Torsional stick-slip behavior in WS2 nanotubes

Strain engineering of Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants: anab initiostudy

We study the influence of mechanical deformations on the Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants from first principles. In particular, we perform symmetry-adapted density functional theory simulations with spin-orbit coupling to determine the variation in the electronic band structure splittings with axial and torsional deformations. We find significant effects in molybdenum and tungsten nanotubes, for which the Zeeman splitting decreases with increase in strain, going to zero for large enough tensile/shear strains, while the Rashba splitting coefficient increases linearly with shear strain, while being zero for all tensile strains, a consequence of the inversion symmetry remaining unbroken. In addition, the Zeeman splitting is relatively unaffected by nanotube diameter, whereas the Rashba coefficient decreases with increase in diameter. Overall, mechanical deformations represent a powerful tool for spintronics in nanotubes.

DFT Study of WS2-Based Nanotubes Electronic Properties under Torsion Deformations

In this study, the influence of torsional deformations on the properties of chiral WS2-based nanotubes was investigated. All calculations presented in this study were performed using the density functional theory (DFT) and atomic gaussian type orbitals basis set. Nanotubes with chirality indices (8, 2), (12, 3), (24, 6) and (36, 9) corresponding to diameters of 10.68 Å, 14.90 Å, 28.26 Å and 41.90 Å, respectively, are examined. Our results reveal that for nanotubes with smaller diameters, the structure obtained through rolling from a slab is not optimal and undergoes spontaneous deformation. Furthermore, this study demonstrates that the nanotube torsion deformation leads to a reduction in the band gap. This observation suggests the potential for utilizing such torsional deformations to enhance the photocatalytic activity of the nanotubes.

Self-Sensing WS2 Nanotube Torsional Resonators

We demonstrate self-sensing tungsten disulfide nanotube (WS₂ NT) torsional resonators. These resonators exhibit all-electrical self-sensing operation with electrostatic excitation and piezoresistive motion detection. We show that the torsional motion of the WS₂ NT resonators results in a change of the nanotube electrical resistance, with the most significant change around their mechanical resonance, where the amplitude of torsional vibrations is maximal. Atomic force microscopy analysis revealed the torsional and bending stiffness of the WS₂ NTs, which we used for modeling the behavior of the WS₂ NT devices. In addition, the solution of the electrostatic boundary value problem shows how the spatial potential and electrostatic field lines around the device impact its capacitance. The results uncover the coupling between the electrical and mechanical behaviors of WS₂ and emphasize their potential to operate as key components in functional devices, such as nanosensors and radio frequency devices.

Outstanding stretchability and thickness-dependent mechanical properties of 2D HfS2, HfSe2, and hafnium oxide

We experimentally determine the elastic properties of 2D HfS₂ and HfSe₂ - two emerging nano-materials whose moderate energy bandgap and dielectric oxidized layer make them highly attractive for functional electronic and optoelectronic systems. We found that the average Young's moduli of HfS₂ and HfSe₂ nano-drumheads are relatively low (45.3 ± 3.7 GPa for a 12.2 nm thick HfS₂ and 39.3 ± 8.9 GPa for a 13.4 nm thick HfSe₂) and depend on the thickness of the nano-drumhead (increasing with thickness for HfS₂ and decreasing for HfSe₂). Moreover, both materials demonstrate outstanding stretchability (fracture strength and maximal strain of 5.7 ± 0.4 GPa and 12.2-14.3%, respectively, for HfS₂; fracture strength and maximal strain of 4.5 ± 1.4 GPa and 14.0-20.9%, respectively, for HfSe₂), which far exceeds the stretchability of other 2D materials and of polymers that are commonly used in flexible electronic applications. Finally, we describe the controlled oxidation of HfSe₂ using a relatively simple laser treatment, which increased the Young's moduli of the thin oxidized layers to 182.6 ± 54.3 GPa. The extraordinary elastic properties of HfS₂ and HfSe₂, together with their excellent electrical and optoelectrical properties, make these 2D materials highly attractive for use in strain engineering and in various stretchable electronic and optoelectronic applications, such as wearable devices.

A new torsion tester based on an electronic autocollimator for characterizing the torsional behaviors of microfibers

Torsional properties of single fibers have considerable effects on the performance of fabrics. A new torsion tester with high resolution is developed based on the torsion-balance method and the autocollimation principle for measuring the torsional properties of single fibers. The tester can measure the torque accurately using the torsion wire of known torsional rigidity. The angular displacement of the torsion wire is measured by a non-contact electronic autocollimator. The calibration results show that the maximum torque capability is around 6 × 10^-6 N m with a theoretical resolution of about 8 × 10^-11 N m. The instrument's validity is verified by comparing the measurement result with that obtained by a torsion pendulum. Torsion tests on four industrially microfibers, including glass fiber, polyimide fiber, carbon fiber, and gold wire, are performed with the tester, which gives shear moduli 71.0 ± 0.4, 5.6 ± 0.1, 22.6 ± 3.1, and 34.3 ± 0.5 GPa, respectively. Young's moduli of these fibers are also provided. It is demonstrated that these fibers have a high anisotropy in mechanical properties.

Torsional strain engineering of transition metal dichalcogenide nanotubes: anab initiostudy

We study the effect of torsional deformations on the electronic properties of single-walled transition metal dichalcogenide (TMD) nanotubes. In particular, considering forty-five select armchair and zigzag TMD nanotubes, we perform symmetry-adapted Kohn-Sham density functional theory calculations to determine the variation in bandgap and effective mass of charge carriers with twist. We find that metallic nanotubes remain so even after deformation, whereas semiconducting nanotubes experience a decrease in bandgap with twist-originally direct bandgaps become indirect-resulting in semiconductor to metal transitions. In addition, the effective mass of holes and electrons continuously decrease and increase with twist, respectively, resulting in n-type to p-type semiconductor transitions. We find that this behavior is likely due to rehybridization of orbitals in the metal and chalcogen atoms, rather than charge transfer between them. Overall, torsional deformations represent a powerful avenue to engineer the electronic properties of semiconducting TMD nanotubes, with applications to devices like sensors and semiconductor switches.

Torsional moduli of transition metal dichalcogenide nanotubes from first principles

We calculate the torsional moduli of single-walled transition metal dichalcogenide (TMD) nanotubes usingab initiodensity functional theory (DFT). Specifically, considering forty-five select TMD nanotubes, we perform symmetry-adapted DFT calculations to calculate the torsional moduli for the armchair and zigzag variants of these materials in the low-twist regime and at practically relevant diameters. We find that the torsional moduli follow the trend: MS₂> MSe₂> MTe₂. In addition, the moduli display a power law dependence on diameter, with the scaling generally close to cubic, as predicted by the isotropic elastic continuum model. In particular, the shear moduli so computed are in good agreement with those predicted by the isotropic relation in terms of the Young's modulus and Poisson's ratio, both of which are also calculated using symmetry-adapted DFT. Finally, we develop a linear regression model for the torsional moduli of TMD nanotubes based on the nature/characteristics of the metal-chalcogen bond, and show that it is capable of making reasonably accurate predictions.

Robot-aided fN∙m torque sensing within an ultrawide dynamic range

In situ scanning electron microscope (SEM) characterization have enabled the stretching, compression, and bending of micro/nanomaterials and have greatly expanded our understanding of small-scale phenomena. However, as one of the fundamental approaches for material analytics, torsion tests at a small scale remain a major challenge due to the lack of an ultrahigh precise torque sensor and the delicate sample assembly strategy. Herein, we present a microelectromechanical resonant torque sensor with an ultrahigh resolution of up to 4.78 fN∙m within an ultrawide dynamic range of 123 dB. Moreover, we propose a nanorobotic system to realize the precise assembly of microscale specimens with nanoscale positioning accuracy and to conduct repeatable in situ pure torsion tests for the first time. As a demonstration, we characterized the mechanical properties of Si microbeams through torsion tests and found that these microbeams were five-fold stronger than their bulk counterparts. The proposed torsion characterization system pushes the limit of mechanical torsion tests, overcomes the deficiencies in current in situ characterization techniques, and expands our knowledge regarding the behavior of micro/nanomaterials at various loads, which is expected to have significant implications for the eventual development and implementation of materials science.

Flatlands in the Holy Land: The Evolution of Layered Materials Research in Israel

The experimental identification of fullerenes in 1985, carbon nanotubes in 1991, inorganic nanotubes in 1992, and graphene in 2004 are cornerstone events that have marked the beginning of the layered nanostructures era of materials science. Nowadays, the synthesis of such low-dimensional systems is a routine practice allowing the controlled fabrication of 0-, 1-, and 2D layered structures of diverse chemical compositions. These systems possess unique physical properties that stem from their structural anisotropy characterized by strong intralayer covalent bonding and weaker interlayer dispersive interactions. This, in turn, results in promising functionality that attracts the attention of scientists from many disciplines including chemists, physicists, material scientists, engineers, as well as life scientists that are interested in both their basic and applied science aspects. Here, a short review of the contribution of the Israeli scientific community to this effort over the past 3 decades, is provided.

Sorting Transition-Metal Dichalcogenide Nanotubes by Centrifugation

Tungsten disulfide (WS₂) nanotubes are cylindrical, multiwall nanotubes with various diameters and wall numbers. They can exhibit various unique properties depending on their structures and thus preparing samples with uniform structures is important for understanding their basic properties and applications. However, most synthesis methods have difficulty to prepare uniform samples, and thus, a purification method to extract nanotubes with a selected diameter and wall number must be developed. Here, we demonstrate a solution-based purification of WS₂ nanotubes using a surfactant solution. Stable dispersions of nanotubes were prepared using nonionic surfactants, which enabled us to sort the diameters and wall numbers of the nanotubes through a centrifugation process. By optimizing the conditions, we successfully obtained thin nanotubes with a mean diameter of 32 nm and mean wall number of 13 with relatively small distributions. Finally, we clarified the relationships between the structure and optical properties of the nanotubes.

Smallest Archimedean Screw: Facet Dynamics and Friction in Multiwalled Nanotubes

We identify a new material phenomenon, where minute mechanical manipulations induce pronounced global structural reconfigurations in faceted multiwalled nanotubes. This behavior has strong implications on the tribological properties of these systems and may be the key to understand the enhanced interwall friction recently measured for boron-nitride nanotubes with respect to their carbon counterparts. Notably, the fast rotation of helical facets in these systems upon coaxial sliding may serve as a nanoscale Archimedean screw for directional transport of physisorbed molecules.

Nanorobotic System iTRo for Controllable 1D Micro/nano Material Twisting Test

In-situ micro/nano characterization is an indispensable methodology for material research. However, the precise in-situ SEM twisting of 1D material with large range is still challenge for current techniques, mainly due to the testing device's large size and the misalignment between specimen and the rotation axis. Herein, we propose an in-situ twist test robot (iTRo) to address the above challenges and realize the precise in-situ SEM twisting test for the first time. Firstly, we developed the iTRo and designed a series of control strategies, including assembly error initialization, triple-image alignment (TIA) method for rotation axis alignment, deformation-based contact detection (DCD) method for sample assembly, and switch control for robots cooperation. After that, we chose three typical 1D material, i.e., magnetic microwire Fe74B13Si11C2, glass fiber, and human hair, for twisting test and characterized their properties. The results showed that our approach is able to align the sample to the twisting axis accurately, and it can provide large twisting range, heavy load and high controllability. This work fills the blank of current in-situ mechanical characterization methodologies, which is expected to give significant impact in the fundamental nanomaterial research and practical micro/nano characterization.

Torsional Resonators Based on Inorganic Nanotubes

We study for the first time the resonant torsional behaviors of inorganic nanotubes, specifically tungsten disulfide (WS₂) and boron nitride (BN) nanotubes, and compare them to that of carbon nanotubes. We have found WS₂ nanotubes to have the highest quality factor (Q) and torsional resonance frequency, followed by BN nanotubes and carbon nanotubes. Dynamic and static torsional spring constants of the various nanotubes were found to be different, especially in the case of WS₂, possibly due to a velocity-dependent intershell friction. These results indicate that inorganic nanotubes are promising building blocks for high-Q nanoelectromechanical systems (NEMS).

Nanotube Electromechanics beyond Carbon: The Case of WS2

The incorporation of nanostructures into nanoelectronic and nanoelectromechanical systems is a long sought-after goal. In the present article, we report the first torsional electromechanical measurements of pure inorganic nanotubes. The WS2 nanotubes exhibited a complex and reproducible electrical response to mechanical deformation. We combined these measurements with density-functional-tight-binding calculations to understand the interplay between mechanical deformation, specifically torsion and tension, and electrical properties of WS2 nanotubes. This yielded the understanding that the electrical response to mechanical deformation may span several orders of magnitude on one hand and detect several modes of mechanical deformation simultaneously on the other. These results demonstrate that inorganic nanotubes could thus be attractive building blocks for nanoelectromechanical systems such as highly sensitive nanometric motion sensors.

Theoretical study of electronic and tribological properties of h-BNC2/graphene, h-BNC2/h-BN and h-BNC2/h-BNC2bilayers

Density functional theory methods are used to investigate the interlayer sliding energy landscape, binding energy and interlayer spacing between h-BNC₂/graphene (I), h-BNC₂/h-BN (II) and h-BNC₂/h-BNC₂(III) bilayer structures.

BCN nanotubes as highly sensitive torsional electromechanical transducers

Owing to their mechanically tunable electronic properties, carbon nanotubes (CNTs) have been widely studied as potential components for nanoelectromechanical systems (NEMS); however, the mechanical properties of multiwall CNTs are often limited by the weak shear interactions between the graphitic layers. Boron nitride nanotubes (BNNTs) exhibit a strong interlayer mechanical coupling, but their high electrical resistance limits their use as electromechanical transducers. Can the outstanding mechanical properties of BNNTs be combined with the electromechanical properties of CNTs in one hybrid structure? Here, we report the first experimental study of boron carbonitride nanotube (BCNNT) mechanics and electromechanics. We found that the hybrid BCNNTs are up to five times torsionally stiffer and stronger than CNTs, thereby retaining to a large extent the ultrahigh torsional stiffness of BNNTs. At the same time, we show that the electrical response of BCNNTs to torsion is 1 to 2 orders of magnitude higher than that of CNTs. These results demonstrate that BCNNTs could be especially attractive building blocks for NEMS.

Revealing the anomalous tensile properties of WS2 nanotubes by in situ transmission electron microscopy

Mechanical properties and fracture behaviors of multiwalled WS2 nanotubes produced by large scale fluidized bed method were investigated under uniaxial tension using in situ transmission electron microscopy probing; these were directly correlated to the nanotube atomic structures. The tubes with the average outer diameter ∼40 nm sustained tensile force of ∼2949 nN and revealed fracture strength of ∼11.8 GPa. Surprisingly, these rather thick WS2 nanotubes could bear much higher loadings than the thin WS2 nanotubes with almost "defect-free" structures studied previously. In addition, the fracture strength of the "thick" nanotubes did not show common size dependent degradation when the tube diameters increased from ∼20 to ∼60 nm. HRTEM characterizations and real time observations revealed that the anomalous tensile properties are related to the intershell cross-linking and geometric constraints from the inverted cone-shaped tube cap structures, which resulted in the multishell loading and fracturing.

Ultrahigh torsional stiffness and strength of boron nitride nanotubes

We report the experimental and theoretical study of boron nitride nanotube (BNNT) torsional mechanics. We show that BNNTs exhibit a much stronger mechanical interlayer coupling than carbon nanotubes (CNTs). This feature makes BNNTs up to 1 order of magnitude stiffer and stronger than CNTs. We attribute this interlayer locking to the faceted nature of BNNTs, arising from the polarity of the B-N bond. This property makes BNNTs superior candidates to replace CNTs in nanoelectromechanical systems (NEMS), fibers, and nanocomposites.

Torsional electromechanical systems based on carbon nanotubes

Carbon nanotubes (CNTs) are among the most highly studied nanomaterials due to their unique (and intertwined) mechanical and electrical properties. Recent advances in fabrication have allowed devices to be fabricated that are capable of applying a twisting force to individual CNTs while measuring mechanical and electrical response. Here, we review major results from this emerging field of study, revealing new properties of the material itself and opening possibilities for advances in future devices.

van der Waals bonding in layered compounds from advanced density-functional first-principles calculations

Although the precise microscopic knowledge of van der Waals interactions is crucial for understanding bonding in weakly bonded layered compounds, very little quantitative information on the strength of interlayer interaction in these materials is available, either from experiments or simulations. Here, using many-body perturbation and advanced density-functional theory techniques, we calculate the interlayer binding and exfoliation energies for a large number of layered compounds and show that, independent of the electronic structure of the material, the energies for most systems are around 20  meV/Å2. This universality explains the successful exfoliation of a wide class of layered materials to produce two-dimensional systems, and furthers our understanding the properties of layered compounds in general.

Direct measurement of the friction between and shear moduli of shells of carbon nanotubes

We report measurements of the shear modulus of each shell and the friction between the two shells of double-shell carbon nanotubes in single nanotube-based nanoelectromechanical devices operated in a transmission electron microscope. In situ nanobeam electron diffraction is applied to obtain the chiral indices of each shell of the nanotube and it allows us to establish a quantitative correlation between the atomic structure and properties of the nanotube under investigation.

Stacking and registry effects in layered materials: the case of hexagonal boron nitride

The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to anchor the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which band gap modulations of ∼0.6  eV are obtained. We propose a simple geometric model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes.

Flux-free conductance modulation in a helical Aharonov--Bohm interferometer

A novel conductance oscillation in a twisted quantum ring composed of a helical atomic configuration is theoretically predicted. The internal torsion of the ring is found to cause a quantum phase shift in the wavefunction that describes the electron's motion along the ring. The resulting conductance oscillation is free from magnetic flux penetrating inside the ring, which is in complete contrast with the case for the ordinary Aharonov-Bohm effect observed in untwisted quantum rings.

Torsion-induced persistent current in a twisted quantum ring

We describe the effects of geometric torsion on the coherent motion of electrons along a thin twisted quantum ring. The geometric torsion inherent in the quantum ring triggers a quantum phase shift in the electrons' eigenstates, thereby resulting in a torsion-induced persistent current that flows along the twisted quantum ring. The physical conditions required for detecting the current flow are discussed.

Helical nanotube structures of MoS2 with intrinsic twisting: an objective molecular dynamics study

Objective molecular dynamics combined with density-functional-based tight binding makes it possible to compute chiral nanotubes as axial-screw dislocations. This enables the surprising revelation of a large catalog of MoS2 nanotubes that lack the prescribed translational symmetry and exhibit chirality-dependent electronic band gaps and elastic constants. Helical symmetry is the natural property to rely on when studying quasi-one-dimensional nanomaterials formally derived or grown via screw dislocations.