Piezoelectricity in 2D van der Waals materials has received considerable interest because of potential applications in nanoscale energy harvesting, sensors, and actuators. However, in all the systems ...studied to date, strain and electric polarization are confined to the basal plane, limiting the operation of piezoelectric devices. In this paper, based on ab initio calculations, we report a 2D materials system, namely, the recently synthesized Janus MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer and multilayer structures, with large out-of-plane piezoelectric polarization. For MXY monolayers, both strong in-plane and much weaker out-of-plane piezoelectric polarizations can be induced by a uniaxial strain in the basal plane. For multilayer MXY, we obtain a very strong out-of-plane piezoelectric polarization when strained transverse to the basal plane, regardless of the stacking sequence. The out-of-plane piezoelectric coefficient d 33 is found to be strongest in multilayer MoSTe (5.7–13.5 pm/V depending on the stacking sequence), which is larger than that of the commonly used 3D piezoelectric material AlN (d 33 = 5.6 pm/V); d 33 in other multilayer MXY structures are a bit smaller, but still comparable. Our study reveals the potential for utilizing piezoelectric 2D materials and their van der Waals multilayers in device applications.
Semiconducting transition metal dichalcogenides (TMDs) are emerging as the potential alternatives to graphene. As in the case of graphene, the monolayer of TMDs can easily be exfoliated using ...mechanical or chemical methods, and their properties can also be tuned. At the same time, semiconducting TMDs (MX2; M = Mo, W and X = S, Se, Te) possess an advantage over graphene in that they exhibit a band gap whose magnitude is appropriate for applications in the opto-electronic devices. Using ab initio simulations, we demonstrate that this band gap can be widely tuned by applying mechanical strains. While the electronic properties of graphene remain almost unaffected by tensile strains, we find TMDs to be sensitive to both tensile and shear strains. Moreover, compared to that of graphene, a much smaller amount of strain is required to vary the band gap of TMDs. Our results suggest that mechanical strains reduce the band gap of semiconducting TMDs causing an direct-to-indirect band gap and a semiconductor-to-metal transition. These transitions, however, significantly depend on the type of applied strain and the type of chalcogenide atoms. The diffuse nature of heavier chalcogenides require relatively more tensile and less shear strain (when the monolayer is expanded in y-direction and compressed in x-direction) to attain a direct-to-indirect band gap transition. In addition, our results demonstrate that the homogeneous biaxial tensile strain of around 10% leads to semiconductor-to-metal transition in all semiconducting TMDs, while through pure shear strain this transition can only be achieved by expanding and compressing the monolayer of MTe2 in the y- and x-directions, respectively. Our results highlight the importance of tensile and pure shear strains in tuning the electronic properties of TMDs by illustrating a substantial impact of the strain on going from MS2 to MSe2 to MTe2.
Two-dimensional (2-D) materials are capable of handling high rates of charge in batteries since metal ions do not need to diffuse in a 3-D lattice structure. However, graphene, which is the most ...well-studied 2-D material, is known to have no Li capacity. Here, adsorption of Li, as well as Na, K, and Ca, on Ti3C2, one representative MXene, is predicted by first-principles density functional calculations. In our study, we observed that these alkali atoms exhibit different adsorption energies depending on the coverage. The adsorption energies of Na, K, and Ca decrease as coverage increases, while Li shows little sensitivity to variance in coverage. This observed relationship between adsorption energies and coverage of alkali ions on Ti3C2 can be explained by their effective ionic radii. A larger effective ionic radius increases interaction between alkali atoms, thus lower coverage is obtained. Our calculated capacities for Li, Na, K, and Ca on Ti3C2 are 447.8, 351.8, 191.8, and 319.8 mAh/g, respectively. Compared to materials currently used in high-rate Li and Na ion battery anodes, MXene shows promise in increasing overall battery performance.
Engineered point defects in two-dimensional (2D) materials offer an attractive platform for solid-state devices that exploit tailored optoelectronic, quantum emission, and resistive properties. ...Naturally occurring defects are also unavoidably important contributors to material properties and performance. The immense variety and complexity of possible defects make it challenging to experimentally control, probe, or understand atomic-scale defect-property relationships. Here, we develop an approach based on deep transfer learning, machine learning, and first-principles calculations to rapidly predict key properties of point defects in 2D materials. We use physics-informed featurization to generate a minimal description of defect structures and present a general picture of defects across materials systems. We identify over one hundred promising, unexplored dopant defect structures in layered metal chalcogenides, hexagonal nitrides, and metal halides. These defects are prime candidates for quantum emission, resistive switching, and neuromorphic computing.
Because of their abundance, sodium and calcium can be attractive in ion batteries for large-scale grid storage. However, many of the anode materials being pursued have limitations including volume ...expansion, lack of passivating films, and slow kinetics. Here, we investigate the adsorption of Na and Ca on graphene with divacancy and Stone–Wales defects in graphene. Our results show that although adsorption of Na and Ca is not possible on pristine graphene, enhanced adsorption is observed on defective graphene because of increased charge transfer between the adatoms and defects. We find that the capacity of graphene increases with the density of the defects. For the maximum possible divacancy defect densities, capacities of 1450 and 2900 mAh/g for Na- and Ca-ion batteries, respectively, can be achieved. For Stone–Wales defects, we find maximum capacities of 1071 and 2142 mAh/g for Na and Ca, respectively. Our results provide guidelines to create better high-capacity anode materials for Na- and Ca-ion batteries.
The unique viscoelastic nature of axons is thought to underlie selective vulnerability to damage during traumatic brain injury. In particular, dynamic loading of axons has been shown to mechanically ...break microtubules at the time of injury. However, the mechanism of this rate-dependent response has remained elusive. Here, we present a microstructural model of the axonal cytoskeleton to quantitatively elucidate the interaction between microtubules and tau proteins under mechanical loading. Mirroring the axon ultrastructure, the microtubules were arranged in staggered arrays, cross-linked by tau proteins. We found that the viscoelastic behavior specifically of tau proteins leads to mechanical breaking of microtubules at high strain rates, whereas extension of tau allows for reversible sliding of microtubules without any damage at small strain rates. Based on the stiffness and viscosity of tau proteins inferred from single-molecule force spectroscopy studies, we predict the critical strain rate for microtubule breaking to be in the range 22–44 s−1, in excellent agreement with recent experiments on dynamic loading of micropatterned neuronal cultures. We also identified a characteristic length scale for load transfer that depends on microstructural properties and have derived a phase diagram in the parameter space spanned by loading rate and microtubule length that demarcates those regions where axons can be loaded and unloaded reversibly and those where axons are injured due to breaking of the microtubules.
Significant efforts have been made in improving the hydrogen evolution reaction (HER) catalytic activity in transition metal dichalcogenides (TMDs), which are promising nonprecious catalysts. ...However, previous attempts have exploited possible solutions to activate the inert basal plane, with little improvement. Among them, the most successful modification requires a careful manipulation of vacancy concentration and strain simultaneously. To fully realize the promise of TMD catalysts for HER in an easier and more effective way, a new means in tuning the HER catalytic activity is needed. Herein, we propose exploiting the inherent structural asymmetry in the recently synthesized family of Janus TMDs as a new means to stimulate HER activity. We report a density functional theory (DFT) study of various Janus TMD monolayers as HER catalysts, and identify the WSSe system as a promising candidate, where the basal plane can be activated without large applied tensile strains and in the absence of significant density of vacancies. We predict that it is possible to realize a strain-free Janus TMD-based catalyst that can readily provide promising intrinsic HER catalytic performance. The calculated density of states and electronic structures reveal that the introduction of in-gap states and a shift in the Fermi level in hydrogen adsorbed systems due to Janus asymmetry is the origin of enhanced HER activity. Our results should pave the way to design high-performance and easy-accessible TMD-based HER catalysts.
We report chemically exfoliated MoS2 nanosheets with a very high concentration of metallic 1T phase using a solvent free intercalation method. After removing the excess of negative charges from the ...surface of the nanosheets, highly conducting 1T phase MoS2 nanosheets exhibit excellent catalytic activity toward the evolution of hydrogen with a notably low Tafel slope of 40 mV/dec. By partially oxidizing MoS2, we found that the activity of 2H MoS2 is significantly reduced after oxidation, consistent with edge oxidation. On the other hand, 1T MoS2 remains unaffected after oxidation, suggesting that edges of the nanosheets are not the main active sites. The importance of electrical conductivity of the two phases on the hydrogen evolution reaction activity has been further confirmed by using carbon nanotubes to increase the conductivity of 2H MoS2.
Substantial research over the past two decades has established that extracellular matrix (ECM) elasticity, or stiffness, affects fundamental cellular processes, including spreading, growth, ...proliferation, migration, differentiation and organoid formation. Linearly elastic polyacrylamide hydrogels and polydimethylsiloxane (PDMS) elastomers coated with ECM proteins are widely used to assess the role of stiffness, and results from such experiments are often assumed to reproduce the effect of the mechanical environment experienced by cells in vivo. However, tissues and ECMs are not linearly elastic materials-they exhibit far more complex mechanical behaviours, including viscoelasticity (a time-dependent response to loading or deformation), as well as mechanical plasticity and nonlinear elasticity. Here we review the complex mechanical behaviours of tissues and ECMs, discuss the effect of ECM viscoelasticity on cells, and describe the potential use of viscoelastic biomaterials in regenerative medicine. Recent work has revealed that matrix viscoelasticity regulates these same fundamental cell processes, and can promote behaviours that are not observed with elastic hydrogels in both two- and three-dimensional culture microenvironments. These findings have provided insights into cell-matrix interactions and how these interactions differentially modulate mechano-sensitive molecular pathways in cells. Moreover, these results suggest design guidelines for the next generation of biomaterials, with the goal of matching tissue and ECM mechanics for in vitro tissue models and applications in regenerative medicine.
To modulate the electronic and optical properties of graphene oxide via controlled deoxidation, a proper understanding of the role of the individual functional group in determining these properties ...is required. We, therefore, have performed ab initio density functional theory based calculations to study the electronic and optical properties of model structures of graphene oxide with different coverages and compositions. In particular, we considered various concentrations of major functional groups like epoxides, hydroxyls, and carbonyls, which mainly consititute the graphene oxide and the reduced graphene oxide. Our calculated electron energy loss spectra (EELS) demonstrate the π plasmon peak to be less sensitive, while π + σ plasmon is found to have a significant blue shift of about 1.0–3.0 eV, when the concentration of epoxy and hydroxyl functional groups in graphene oxide vary from 25% to 75%. However, the increase in carbonyl groups in the center of the graphene sheet creates holes, which lead to the red shift of the EELS. In the case of 37.5% of oxygen-to-carbon ratio, we find the π plasmon peak to be shifted by roughly 1.0 eV as compared to that of the pristine graphene. Our results agree well with the experimental findings which suggest a blue shift in the EELS of graphene oxide and an absorption feature due to a π electron transition of the carbonyl groups at a lower energy than that of epoxy and hydroxyl groups. We also show that the increase in the width of the hole created by the carbonyl groups significantly decreases the optical gap and opens the band gap, and thus, we argue that reduced graphene oxide with mostly carbonyl groups could be a useful material for developing tunable opto-electronic nanodevices.
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