Nanodiodes based on double-walled transition metal dichalcogenide nanotubes: enhancement of current rectification via a moiré superlattice

In order to develop a rotary driving element with a shape-memory alloy (SMA) thin strip, the torsional deformation property, cyclic deformation property and torsional fatigue property of a TiNi SMA thin strip were investigated. The results obtained can be summarized as follows. (1) During twisting the SMA thin strip, the martensitic transformation appears along the edge of the thin strip and develops toward the central part. The rate of increase in the tensile strain of the edge increases gradually with an increase in angle of twist. (2) Torque increases with an increase in angle of twist. The torsional deformation properties change slightly under cyclic torsion. (3) With respect to the fatigue properties, the number of cycles to failure decreases with an increase in the angle of twist. The fatigue life in pulsating torsion is five times longer than that in alternating torsion at the same angle of twist. (4) The two-way rotary movement of an opening and closing door model was demonstrated. Therefore, a rotary driving element with a small and simple mechanism can be developed by using the SMA thin strip.

Twisting of the acene backbone out of planarity in twisted acenes leads to a variation in their optical and electronic properties. The effect of increasing twist angles on the properties of the photoexcited triplet states of a series of anthracene-based helically tethered twisted acenes is investigated here by Electron Paramagnetic Resonance (EPR) spectroscopy. Increasing signal intensities with increasing twist angles indicate increased intersystem crossing efficiencies for the twisted molecules compared to the untethered reference compound. Variations in the electron spin polarisation observed in the transient EPR spectra, in particular for the compound with the shortest tether, imply changes in the sublevel population kinetics depending on molecular geometry. Changes in the zero-field splitting parameters and in the proton hyperfine couplings for compounds with short tethers and therefore higher twist angles point towards a slight redistribution of the spin density compared to the parent compound. The experimental results can be explained by considering both an increase in twist angle and a related decrease in the dihedral angle between the phenyl side groups and the acene core. The observation of a clear excitation-wavelength dependence suggests preferential excitation of different molecular conformations, with conformers characterised by higher twist angles selected at higher wavelengths.

Electronic structures of twist-stacked 1T-TaS2 bilayers

The interlayer coupling in van der Waals heterostructures governs a variety of optical and electronic properties. The intrinsic dipole moment of Janus transition metal dichalcogenides (TMDs) offers a simple and versatile approach to tune the interlayer interactions. In this work, we demonstrate how the van der Waals interlayer coupling and charge transfer of Janus MoSSe/MoS2 heterobilayers can be tuned by the twist angle and interface composition. Specifically, the Janus heterostructures with a sulfur/sulfur (S/S) interface display stronger interlayer coupling than the heterostructures with a selenium/sulfur (Se/S) interface as shown by the low-frequency Raman modes. The differences in interlayer interactions are explained by the interlayer distance computed by density-functional theory (DFT). More intriguingly, the built-in electric field contributed by the charge density redistribution and interlayer coupling also play important roles in the interfacial charge transfer. Namely, the S/S and Se/S interfaces exhibit different levels of photoluminescence (PL) quenching of MoS2 A exciton, suggesting enhanced and reduced charge transfer at the S/S and Se/S interface, respectively. Our work demonstrates how the asymmetry of Janus TMDs can be used to tailor the interfacial interactions in van der Waals heterostructures.

Funding Acknowledgements None Hypertrophic obstructive cardiomyopathy (HOCM) is characterized by muscle hypertrophy and fibrosis, interfering with force generation and relaxation. Abnormal ventricular (LV) myocardial deformation have been demonstrated in patients with HOCM at rest, but there is lack of data regarding the deformational mechanics in exercise. AIMS: We wanted to assess the adaptability of LV deformational behaviour to physical exercise in HCM patients as compared to healthy control subjects. METHODS 24 obstructive HOCM (age 51.2 ± 14.2yrs; 16 men, LVOT-obstruction 56 ± 19mmHg at rest or on Valsalva maneuver) and 32 control subjects (50.9 ± 6.8 yrs, 19 men from the MAGYAR-PATH Registry) underwent supine bicycle stress echocardiography (ESE) with measurements of 2D- and Doppler, 2D-Speckle Tracking Tracking Imaging and 3D-Full Volume Analysis. Beyond conventional LV/RV functional measurements; peak longitudinal (LS), circumferential (CS) Strain values; peak Twist and Torsion angles; post-systolic shortening index (PSS) and diastolic phase indices (Untwisting time - UTT and rate - UTR) calculated; the UTR/Twist ratio was given as "coupling index". The LV/RV-EF calculated by 3D-Full Volume Analysis off-line by TomTec Arena™ software both at rest and on submaximal ESE. RESULTS The HOCM group had lower resting LS (-14.6 ± 4.5 vs 18.4 ± 2.6%, p < 001) but higher CS (-32.9 ± 5.1 vs 28.8 ± 2.3%, p < 0.001) and Twist angles (9.9 ± 2.6 vs 6.1 ± 2.2º, p < 0.01) than control subjects. Exercise induced an increase in all strains in control subjects, but much less in HOCM (LS: -21.4 ± 3.5 vs 15.1 ± 3.0% and CS: -33.9 ± 3.6 vs 34.1 ± 4.2% in HOCM, p < 0.02 in controls, NS in HOCM); the increase of Twist angle was minimal in HOCM (Δ1.2 ± 1.2 vs Δ3.6 ± 2.3º in controls, p < 0.01). The PSS was more pronounced on ESE in HOCM than in controls (46.6 ± 12 vs 21.2 ± 9.6% in controls, p < 0.001). Peak UTR was slower (118 ± 2.1 vs 133.1 ± 14.1 º/s) during ESE and occured later (141 ± 19 vs 121 ± 9.1% of systolic time, p < 0.02) in HOCM than in controls. There was significant relationship between the Twist and UTR in control subjects (β=-0.0807, p < 0.001), but not in HOCM (β=-0.0046, p = 0.05). The UTR/Twist ratio diminished only in HOCM but not in controls (-8.0 ± 0.6 vs -13.1 ± 2.5 1/s, p < 0.02). CONCLUSIONS The HOCM patients had significantly impaired strain-adaptability; developed post-systolic shortening and no LV Torsional reserve was found on exercise. Also, I detected decreased and delayed UTR indices in the HOCM group. These findings support evidence for reduced systolic-diastolic coupling efficiency, assessed by Twist-Untwist mechanics in HOCM patients, which can contribute to the development of exercise-related symptoms and the dynamic LVOT-obstruction. This unique pattern of deformational behaviour to exercise can help in the differential diagnostic workup in patients with LV hypertrophy of unknown aetiology and also would hold additional value in the risk stratification process for patients with HCM-phenotypes.

We present a general procedure to introduce electronic polarization into classical Molecular Dynamics (MD) force fields using a Neural Network (NN) model. We apply this framework to the simulation of a solid-liquid interface where the polarization of the surface is essential to correctly capture the main features of the system. By introducing a multi-input, multi-output NN and treating the surface polarization as a discrete classification problem, we are able to obtain very good accuracy in terms of quality of predictions. Through the definition of a custom loss function we are able to impose a physically motivated constraint within the NN itself making this model extremely versatile, especially in the modeling of different surface charge states. The NN is validated considering the redistribution of electronic charge density within a graphene based electrode in contact with an aqueous electrolyte solution, a system highly relevant to the development of next generation low-cost supercapacitors. We compare the performances of our NN/MD model against Quantum Mechanics/Molecular Dynamics simulations where we obtain a most satisfactory agreement.

To improve the efficiency of perovskite based solar cells, doping of heavier elements in Perovskite materials (ABX3) can modulate its electronic and optical properties significantly. Thus it is important to understand the possible microscopic origin of the band gap modification and optical enhancement after heavier element doping using first-principles studies. Here we investigate the effect of La doping, while substituting the Ca atom, on the electronic and optical properties in CaTiO3 perovskite material using generalized gradient approximation within density functional theory. We observe a decrease in lattice constants and bond lengths in LaxCa1−xTiO3, mainly due to re-distribution of electronic charge density between La and Oxygen, as confirmed by charge density contour. We further notice a widening of electronic band gap and an upward shift of Fermi level into the conduction band, thus characterizing LaxCa1−xTiO3 as an n-type material. DOS diagram attributes this shift mainly due to the appearance of La p-DOS and d-DOS and their repulsion with N p-DOS, when La enters into the host lattice at Ca site. Investigation of optical properties upon La Doping in CaTiO3 exhibits further shifting of polarization and refractive index to lower values as compared to its pure counterpart, due to dominating semiconducting behavior and hence one observes a blue shift in absorption and reflection spectrum accordingly. Energy loss function is found to be consistent with absorption and extinction coefficient measured in case of LaxCa1−xTiO3. All these results are found to be consistent with the existing experimental and first-principles studies.

ABSTRACTUltrafast atomic dynamics induced by electronic and optical excitation opens new possibilities for functionalization of two-dimensional and layered materials. Understanding the impact of perturbed valence band populations on both the strong covalent bonds and relatively weaker van der Waals interactions is important for these anisotropic systems. While the dynamics of strong covalent bonds has been explored both experimentally and theoretically, relatively fewer studies have focused on the impact of excitation on weak bonds like van der Waals and hydrogen-bond interactions. We perform non-adiabatic quantum molecular dynamics (NAQMD) simulations to study photo-induced dynamics in MoS2 bilayer. We observe photo-induced non-thermal contraction of the interlayer distance in the MoS2 bilayer within 100 femtoseconds after photoexcitation. We identify a large photo-induced redistribution of electronic charge density, whose Coulombic interactions could explain the observed inter-layer contraction.

The flexoelectric effect is the coupling between strain, polarization, and their gradients, which are prominent at the nanoscale. Although this effect is important to understand nanostructures, such as domain walls in ferroelectrics, its electronic mechanism is not clear. In this work, we combined phase-field simulations and first-principles calculations to study the 180° domain walls in tetragonal ferroelectric PbTiO3 and found that the source of Néel components is the gradient of the square of spontaneous polarization. Electronic structural analysis reveals that there is a redistribution of electronic charge density and potential around domain walls, which produces the electric field and Néel components. This work thus sheds light on the electronic mechanism of the flexoelectric effect around 180° domain walls in tetragonal ferroelectrics.

We perform a comparative study for the quantum transport of telescoping carbon nanotubes, where the (5,5) and (10,10) nanotubes are coaxially aligned, using first-principles local-density-functional and tight-binding calculations. In both calculations, the intertube conductance initially increases as the hybridized length in the contact region increases, and then decreases, exhibiting a maximum conductance. However, the calculated conductances from first principles are generally smaller than those from the single $\ensuremath{\pi}$-orbital tight-binding model. In the first-principles calculations, we obtain the maximum intertube conductance that does not exceed ${G}_{0}\phantom{\rule{0.3em}{0ex}}(=2{e}^{2}∕h)$, while individual tubes have two conducting channels, giving the conductance of $2{G}_{0}$. On the other hand, the single $\ensuremath{\pi}$-orbital tight-binding model gives the maximum conductance close to $2{G}_{0}$, similar to previous calculations. Using a double-wall nanotube, we examine the effect of interwall interactions on conductance and find that the ${\ensuremath{\pi}}^{*}$ states of the inner and outer tubes are strongly coupled in the tight-binding model, allowing for an extra conducting channel, while the ${\ensuremath{\pi}}^{*}$ channel is closed in the first-principles calculations.

Investigating the performance of a twisted modified Savonius rotor

Two-dimensional (2D) materials are attractive for their unique electronic structures and electrocatalytic properties. In this work, we propose to use the twist angle as a knob to tune the electrocatalytic properties of 2D materials. As proof of concept, we investigate the effects of twist angle (>10°) on the electrocatalytic properties of twisted bilayer graphene (tBLG). We predict the activity of tBLG with the twist angle of 13.174° and 21.787° for the hydrogen evolution reaction (HER) using a density functional theory (DFT) calculation and computational hydrogen electrode (CHE) approach. We calculate the hydrogen adsorption energy (ΔGH*) at various sites on tBLG and examine their angle-dependency. By comparing the ΔGH* for different active sites of untwisted bilayer graphene (BLG) and tBLG, we find that the ΔGH* decreases with the increase of the twist angle. As a result, the thermodynamic limiting potential for the HER increases with the twist angle. Furthermore, the ΔGH* shows a correlation with the layer distance (d̄) and the site location on the 2D plane. Detailed analysis reveals that the twist of bilayer graphene could increase the z height (dz) of the active sites as a function of their distance to the symmetry centers, alter the local geometry of the active sites, and therefore modify the ΔGH*. These results indicate that the twist angle can be effectively used as a knob to fine-tune the electrocatalytic properties of 2D materials.

In this paper, a device with high accuracy capacitive sensor (with the error of 0.1 micrometer) is constructed to measure the axial thermal expansion coefficent of the twisted carbon fibers and yarns of Kevlar. A theoretical model based on the thermal elasticity and the geometrical features of the twisted structure is also presented to predict the axial expansion coefficient. It is found that the twist angle, diameter and pitch have remarkable influences on the axial thermal expansion coefficients of the twisted carbon fibers and Kevlar strands, and the calculated results are in good agreement with experimental data. We found that, with the increase of the twist angle, the absolute value of the axial thermal expansion coefficient increases. For the Kevlar samples, the expansion coefficient will grow by about 46% when the twist angle increases from 0 to 25 degrees, while the carbon fiber samples will grow by about 72% when the twist angle increases from 0 to 35 degrees. The experimental measurements and the model calculations reveal important properties of the thermal expansion in the twisted structures. Most notably, the expansion of the strand during heating or cooling can be zero when the twist angle is around β = arcsin(αL/αT)^1/2, where β denotes twist angle of the strand and αL, αT are the longitute and the transverse thermal expansion coefficient of the strand, respectively. According to the present experiments and analyses, a method to control the axial thermal expansion coefficient of this new kind of twisted structure is proposed. Moreover, the mechanism of this tunable thermal expansion is discussed. Based on the model, a method that can be used to rectify the thermal expansion properties of the twist structures is established. This may be a new way of fabricating zero expansion composite materials in the future.

Probing the stochastic fracture behavior of twisted bilayer graphene: Efficient ANN based molecular dynamics simulations for complete probabilistic characterization

We investigate the generation of soliton-like pulses along a DNA chain which takes into account both torsional and solvent interaction effects. Interactions between neighboring base pairs are described by a twist angle. Twisting is essential in the model to capture the importance of nonlinear effects for the thermodynamical properties. The nonlinear dynamics of the DNA is then modeled in the Hamiltonian approach by the generalized Dauxois–Peyrard–Bishop model (combination of several models). We introduce the generalized discrete nonlinear Schrödinger equation describing the dynamics of modulated wave through the twisted DNA with solvent interaction. The modulational instability is studied and we present an analytical expression for the MI gain to show the effects of twist angle on MI gain spectra as well as on stability diagram. With the increase of the twist angle the MI gain decreases then increases. Some interesting MI phenomena appear with an additional new MI region as the twist angle increases. The instability and stability diagrams are also affected. Numerical simulations are carried out to show the validity of the analytical approach. The result is that the initial wave breaks into a train of ultrashort pulses with repetition rates, which are trapped in some sites. The impact of the twist angle is investigated and we obtain that the twist angle affects the dynamics of stable patterns generated through the molecule. Thereafter, we study energy localization in the framework of twisted DNA with solvent interaction. While the twist angle leads to a stronger localization of energy, the solvent interaction delocalizes energy along the molecule.