Smallest Archimedean Screw: Facet Dynamics and Friction in Multiwalled Nanotubes

Direct growth of single-chiral-angle tungsten disulfide nanotubes using gold nanoparticle catalysts

Transition metal dichalcogenide (TMD) nanotubes offer a unique platform to explore the properties of TMD materials at the one-dimensional limit. Despite considerable efforts thus far, the direct growth of TMD nanotubes with controllable chirality remains challenging. Here we demonstrate the direct and facile growth of high-quality WS2 and WSe2 nanotubes on Si substrates using catalytic chemical vapour deposition with Au nanoparticles. The Au nanoparticles provide unique accommodation sites for the nucleation of WS2 or WSe2 shells on their surfaces and seed the subsequent growth of nanotubes. We find that the growth mode of nanotubes is sensitive to the temperature. With careful temperature control, we realize ~79% WS2 nanotubes with single chiral angles, with a preference of 30° (~37%) and 0° (~12%). Moreover, we demonstrate how the geometric, electronic and optical properties of the synthesized WS2 nanotubes can be modulated by the chirality. We anticipate that this approach using Au nanoparticles as catalysts will facilitate the growth of TMD nanotubes with controllable chirality and promote the study of their interesting properties and applications.

Nanotube Slidetronics

One-dimensional slidetronics is predicted for double-walled boron-nitride nanotubes. Local electrostatic polarization patterns along the body of the nanotube are found to be determined by the nature of the two nanotube walls, their relative configuration, and circumferential faceting modulation during coaxial interwall sliding. By careful choice of chiral indices, chiral polarization patterns can emerge that spiral around the nanotube circumference. The potential usage of the discovered slidetronic effect for low-dimensional nanogenerators is briefly discussed.

Effect of strain engineering on superlubricity in a double-walled carbon nanotube

Double-walled carbon nanotubes (DWCNTs) have received a great deal of attention due to their great potential in the field of superlubricity. However, this superlubricity is susceptible to failure in practical applications due to the introduction of various defects. Here, a novel method based on strain engineering is employed for achieving superlubricity in the DWCNT using molecular dynamics simulations. The DWCNT exhibits a superlow friction force when an inner tube slides against a stretched outer tube even with a low content of defects. However, strain engineering shows its limitation on superlubricity in the case of a large magnitude of strain or a high content of point defects. The mechanism of superlubricity in the DWCNT could be explained by the analysis of the energy barrier.

Structural lubricity in soft and hard matter systems

Over the recent decades there has been tremendous progress in understanding and controlling friction between surfaces in relative motion. However the complex nature of the involved processes has forced most of this work to be of rather empirical nature. Two very distinctive physical systems, hard two-dimensional layered materials and soft microscopic systems, such as optically or topographically trapped colloids, have recently opened novel rationally designed lines of research in the field of tribology, leading to a number of new discoveries. Here, we provide an overview of these emerging directions of research, and discuss how the interplay between hard and soft matter promotes our understanding of frictional phenomena.

Structural lubricity is one of the most interesting concepts in modern tribology, which promises to achieve ultra-low friction over a wide range of length-scales. Here the authors highlight novel research lines in this area achievable by combining theoretical and experimental efforts on hard two-dimensional materials and soft colloidal and cold ion systems.

Friction of physisorbed nanotubes: rolling or sliding?

The structure and motion of carbon and h-BN nanotubes (NTs) deposited on graphene is inquired theoretically by simulations based on state-of-the-art interatomic force fields. Results show that any typical cylinder-over-surface approximation is essentially inaccurate. NTs tend to flatten at the interface with the substrate and upon driving they can either roll or slide depending on their size and on their relative orientation with the substrate. In the epitaxially aligned orientation we find that rolling is always the main mechanism of motion, producing a kinetic friction linearly growing with the number of walls, in turn causing an unprecedented supra-linear scaling with the contact area. A 30 degrees misalignment raises superlubric effects, making sliding favorable against rolling. The resulting rolling-to-sliding transition in misaligned NTs is explained in terms of the faceting appearing in large multi-wall tubes, which is responsible for the increased rotational stiffness. Modifying the geometrical conditions provides an additional means of drastically tailoring the frictional properties in this unique tribological system.

Nanomechanics of few-layer materials: do individual layers slide upon folding?

Folds naturally appear on nanometrically thin materials, also called "2D materials", after exfoliation, eventually creating folded edges across the resulting flakes. We investigate the adhesion and flexural properties of single-layered and multilayered 2D materials upon folding in the present work. This is accomplished by measuring and modeling mechanical properties of folded edges, which allows for the experimental determination of the bending stiffness (κ) of multilayered 2D materials as a function of the number of layers (n). In the case of talc, we obtain κ ∝ n ³ for n ≥ 5, indicating no interlayer sliding upon folding, at least in this thickness range. In contrast, tip-enhanced Raman spectroscopy measurements on edges in folded graphene flakes, 14 layers thick, show no significant strain. This indicates that layers in graphene flakes, up to 5 nm thick, can still slip to relieve stress, showing the richness of the effect in 2D systems. The obtained interlayer adhesion energy for graphene (0.25 N/m) and talc (0.62 N/m) is in good agreement with recent experimental results and theoretical predictions. The obtained value for the adhesion energy of graphene on a silicon substrate is also in agreement with previous results.

High Density Static Charges Governed Surface Activation for Long-Range Motion and Subsequent Growth of Au Nanocrystals

How a heavily charged metal nanocrystal, and further a dual-nanocrystals system behavior with continuous electron charging? This refers to the electric dynamics in charged particles as well as the crystal growth for real metal particles, but it is still opening in experimental observations and interpretations. To this end, we performed an in-situ electron-beam irradiation study using transmission electron microscopy (TEM) on the Au nanocrystals that freely stand on the nitride boron nanotube (BNNT). Au nanocrystalline particles with sizes of 2–4 nm were prepared by a well-controlled sputtering method to stand on the BNNT surface without chemical bonding interactions. Au nanoparticles presented the surface atomic disorder, diffusion phenomena with continuous electron-beam irradiation, and further, the long-range motion that contains mainly the three stages: charging, activation, and adjacence, which are followed by final crystal growth. Firstly, the growth process undergoes the lattice diffusion and subsequently the surface-dominated diffusion mechanism. These abnormal phenomena and observations, which are fundamentally distinct from classic cases and previous reports, are mainly due to the overcharging of Au nanoparticle that produces a surface activation state in terms of high-energy plasma. This work therefore brings about new observations for both a single and dual-nanocrystals system, as well as new insights in understanding the resulting dynamics behaviors.

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.