Polarization tunable bidirectional photoresponse in Van der Waals α − In 2 Se 3 / Nb X 2
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Ferroelectric diodes can generate a polarization-controlled bidirectional photoresponse to simulate inhibition and promotion behaviors in the artificial neuromorphic system with fast speed, high energy efficiency, and nonvolatility. However, the existing ferroelectric diodes based on ferroelectric oxides suffer from a weak bidirectional photoresponse (below 1 mA/W), difficult miniaturization, and a large response photon energy (over 3 eV). Here, we design a series of van der Waals $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}\text{/}\mathrm{Nb}{X}_{2}$ ($X$ = S, Se, and Te) ferroelectric diodes with bidirectional photoresponse by using ab initio quantum transport simulation. These devices show a maximum bidirectional photoresponse of 30 (\ensuremath{-}19) mA/W and a minimum response photon energy of 1.3 eV at the monolayer thickness. Our work shows advanced optoelectronic applications of the van der Waals ferroelectric diodes in the future artificial neuromorphic system.Electrostatics
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Electrostatic force microscope
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We have measured the effect of a depolarizing field on the properties of a ferroelectric capacitor. By systematically adjusting the amount of charge available to compensate the polarization, we can control the strength of the field inside the ferroelectric. We find that even a few percent of uncompensated polarization charge results in a significant suppression of measured ferroelectric properties, and a complete lack of compensating charge leaves a greatly reduced, although nonzero, polarization. The effect of a depolarizing field is briefly discussed in terms of proposed ferroelectric device applications.
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Van der Waals interactions, primarily attractive van der Waals interactions, have been studied over one and half centuries. However, repulsive van der Waals interactions are less widely studied than attractive van der Waals interactions. In this article, we focus on repulsive van der Waals interactions. Van der Waals interactions are dipole–dipole interactions. In this article, we study the van der Waals interactions between multiple dipoles. Specifically, we focus on two-dimensional six-body van der Waals interactions. This study has many potential applications. For example, the result may be applied to physics, chemistry, chemical engineering, and other fields of sciences and engineering, such as breaking molecules.
Hamaker constant
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Electrostatics
DLVO theory
Particle (ecology)
Hamaker constant
Surface force
Electrostatic force microscope
Electrostatic interaction
Force Field
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A mathematical model for analyzing the van der Waals interaction between the internal aqueous droplets (W1) and the external aqueous phase (W2) of double emulsions has been established. The effects of Hamaker constants of the materials forming the system, especially those of the two different adsorbed surfactant layers with uniform density (A1 and A2), on the van der Waals interaction were investigated. The overall van der Waals interaction across the oil film is a combined result of four individual parts, that is, W1−W2, A1−A2, W1−A1, and A2−W2 van der Waals interaction, and it may be either attractive or repulsive depending on many factors. It was found that the overall van der Waals interaction is dominated by the W1−W2 interaction at large separation distances between the W1/O and O/W2 interfaces, while it is mostly determined by the A1−A2 interaction when the two interfaces are extremely close. Specifically, in the cases when the value of the Hamaker constant of the oil phase is intermediate between those of W1 and W2 and there is a thick oil film separating the two interfaces, a weak repulsive overall van der Waals interaction will prevail. If the Hamaker constant of the oil phase is intermediate between those of A1 and A2 and the two interfaces are very close, the overall van der Waals interaction will be dominated by the strong repulsive A1−A2 interaction. The repulsive van der Waals interaction at such cases helps stabilize the double emulsions.
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It is shown that the constant current method, in which a dielectric is charged with a constant current while the voltage is monitored, allows one to determine the dependence of the stable ferroelectric polarization with the electric field. The determination is based on two successive experiments separated in time by a short-circuit period: a charging process in which polarization switching occurs followed by a recharging with the same current polarity. Analysis of the recharging experiments for poly(vinylidene fluoride), PVDF, shows that the polarization appearing in it is a metastable ferroelectric polarization, due to the reorientation of ferroelectric polarization lost during the short-circuit period. The method was applied to measure the ferroelectric polarization in PVDF samples with different β-phase contents and in an exploratory way for a few other ferroelectric polymers.
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Valley polarization and ferroelectricity are the two basic concepts in electronic device applications. However, the coexistence of these two scenarios in one material has not been reported. Here, using first-principles calculations, we demonstrated that the two-dimensional GaAsC6 monolayer which is a hybrid structure of GaAs and graphene has a pair of inequivalent valleys with opposite Berry curvatures and an intrinsic out-of-plane spontaneous electric polarization. It also has a direct band gap of about 1.937 eV and a high carrier mobility of about 1.80 × 105 cm2 V-1 s-1, which are promising for electronic device applications. The integration of valley polarization and ferroelectricity in a single material offers a promising platform for the design of electronic devices.
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The van der Waals volume is a widely used descriptor in modeling physicochemical properties. However, the calculation of the van der Waals volume (V(vdW)) is rather time-consuming, from Bondi group contributions, for a large data set. A new method for calculating van der Waals volume has been developed, based on Bondi radii. The method, termed Atomic and Bond Contributions of van der Waals volume (VABC), is very simple and fast. The only information needed for calculating VABC is atomic contributions and the number of atoms, bonds, and rings. Then, the van der Waals volume (A(3)/molecule) can be calculated from the following formula: V(vdW) = summation operator all atom contributions - 5.92N(B) - 14.7R(A) - 3.8R(NR) (N(B) is the number of bonds, R(A) is the number of aromatic rings, and R(NA) is the number of nonaromatic rings). The number of bonds present (N(B)) can be simply calculated by N(B) = N - 1 + R(A) + R(NA) (where N is the total number of atoms). A simple Excel spread sheet has been made to calculate van der Waals volumes for a wide range of 677 organic compounds, including 237 drug compounds. The results show that the van der Waals volumes calculated from VABC are equivalent to the computer-calculated van der Waals volumes for organic compounds.
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Van der Waals interactions are always present in handling micro objects and will influence the whole manipulation process. This paper investigates the effects of van der Waals forces on the design and planning of micromanipulation. Origins of van der Waals interactions are shown first. Van der Waals forces between micro objects of several typical configurations in micromanipulation process are characterized. The related aspects of van der Waals forces are discussed based on the theoretical analysis. Methods of control adhesion induced by van der Waals forces in micromanipulation processes are presented.
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