Torsional Deformations in Subnanometer MoS Interconnecting Wires

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Mapping 1D Confined Electromagnetic Edge States in 2D Monolayer Semiconducting MoS2 Using 4D-STEM

Four-dimensional (4D) scanning transmission electron microscopy is used to study the electric fields at the edges of 2D semiconducting monolayer MoS₂. Sub-nanometer 1D features in the 2D electric field maps are observed at the outermost region along zigzag edges and also along nanowire MoS-terminated MoS₂ edges. Atomic-scale oscillations are detected in the magnitude of the 1D electromagnetic edge state, with spatial variations that depend on the specific periodic edge reconstructions. Electric field reconstructions, along with integrated differential phase contrast reconstructions, reveal the presence of low Z number atoms terminating many of the uniform edges, which are difficult to detect by annular dark field scanning transmission electron microscopy due to its limited dynamic range. Density functional theory calculations support the formation of periodic 1D edge states and also show that enhancement of the electric field magnitude can occur for some edge terminations. The experimentally observed electric fields at the edges are attributed to the absence of an opposing electric field from a nearest neighbor atom when the electron beam propagates through the 2D monolayer and interacts. These results show the potential of 4D-STEM to map the atomic scale structure and fluctuations of electric fields around edge atoms with different bonding states than bulk atoms in 2D materials, beyond conventional imaging.

Atomic-number (Z)-correlated atomic sizes for deciphering electron microscopic molecular images

Atomic resolution transmission electron microscopy (TEM) has opened up a new era of molecular science by providing atomic video images of dynamic motions of single organic and inorganic molecules. However, the images often look different from the images of molecular models, because these models are designed to visualize the electronic properties of the molecule instead of nuclear electrostatic potentials that are felt by the e-beam in TEM imaging. Here, we propose a molecular model that reproduces TEM images using atomic radii correlated to atomic number (Z). The model serves to provide a priori a useful idea of how a single molecule, molecular assemblies, and thin crystals of organic or inorganic materials look in TEM.

With the advent of atomic resolution transmission electron microscopy (AR-TEM) achieving sub-Ångstrom image resolution and submillisecond time resolution, an era of cinematic molecular science where chemists can visually study the time evolution of molecular motions and reactions at atomistic precision has arrived. However, the appearance of experimental TEM images often differs greatly from that of conventional molecular models, and the images are difficult to decipher unless we know in advance the structure of the specimen molecules. The difference arises from the fundamental design of the molecular models that represent atomic connectivity and/or the electronic properties of molecules rather than the nuclear charge of atoms and electrostatic potentials that are felt by the e-beam in TEM imaging. We found a good correlation between the atomic number (Z) and the atomic size seen in TEM images when we consider shot noise in digital images. We propose Z-correlated (ZC) atomic radii for modeling AR-TEM images of single molecules and ultrathin crystals with which we can develop a good estimate of the molecular structure from the TEM image much more easily than with conventional molecular models. Two parameter sets were developed for TEM images recorded under high-noise (ZC_HN) and low-noise (ZC_LN) conditions. The molecular models will stimulate the imaginations of chemists planning to use AR-TEM for their research.

Linear and Helical Cesium Iodide Atomic Chains in Ultranarrow Single-Walled Carbon Nanotubes: Impact on Optical Properties

One-dimensional (1D) atomic chains of CsI were previously reported in double-walled carbon nanotubes with ∼0.8 nm inner diameter. Here, we demonstrate that, while 1D CsI chains form within narrow ∼0.73 nm diameter single-walled carbon nanotubes (SWCNTs), wider SWCNT tubules (∼0.8-1.1 nm) promote the formation of helical chains of CsI 2 × 1 atoms in cross-section. These CsI helices create complementary oval distortions in encapsulating SWCNTs with highly strained helices formed from strained Cs₂I₂ parallelogram units in narrow tubes to lower strain Cs₂I₂ units in wider tubes. The observed structural changes and charge distribution were analyzed by density-functional theory and Bader analysis. CsI chains also produce conformation-selective changes to the electronic structure and optical properties of the encapsulating tubules. The observed defects are an interesting variation from defects commonly observed in alkali halides as these are normally associated with the Schottky and Frenkel type. The energetics of CsI 2 × 1 helix formation in SWCNTs suggests how these could be controllably formed.

Intrinsic helical twist and chirality in ultrathin tellurium nanowires

Robust atomic-to-meso-scale chirality is now observed in the one-dimensional form of tellurium. This enables a large and counter-intuitive circular-polarization dependent second harmonic generation response above 0.2 which is not present in two-dimensional tellurium. Orientation variations in 1D tellurium nanowires obtained by four-dimensional scanning transmission electron microscopy (4D-STEM) and their correlation with unconventional non-linear optical properties by second harmonic generation circular dichroism (SHG-CD) uncovers an unexpected circular-polarization dependent SHG response from 1D nanowire bundles - an order-of-magnitude higher than in single-crystal two-dimensional tellurium structures - suggesting the atomic- and meso-scale crystalline structure of the 1D material possesses an inherent chirality not present in its 2D form; and which is strong enough to manifest even in the aggregate non-linear optical (NLO) properties of aggregates.

Efficient growth and characterization of one-dimensional transition metal tellurides inside carbon nanotubes

Atomically thin one-dimensional (1D) van der Waals wires of transition metal monochalocogenides (TMMs) have been anticipated as promising building blocks for integrated nanoelectronics. While reliable production of TMM nanowires has eluded scientists over the past few decades, we finally demonstrated a bottom-up fabrication of MoTe nanowires inside carbon nanotubes (CNTs). Still, the current synthesis method is based on vacuum annealing of reactive MoTe₂, and limits access to a variety of TMMs. Here we report an expanded framework for high-yield synthesis of the 1D tellurides including WTe, an previously unknown family of TMMs. Experimental and theoretical analyses revealed that the choice of suitable metal oxides as a precursor provides a useful yield for their characterization. These TMM nanowires exhibit a significant optical absorption in the visible-light region. More important, electronic properties of CNTs can be tuned by encapsulating different TMM nanowires.

1D topological phases in transition-metal monochalcogenide nanowires

The Su-Schrieffer-Heeger (SSH) model is a prototypical one-dimensional (1D) diatomic lattice model for non-trivial topological phases and topological excitations. Theoretically, many variations and extensions of the SSH model have been proposed and explored to better understand the novel aspects of topological physics in low dimensions on the nanoscale. However, the outstanding challenge remains to find real nanomaterials with robust structural stability for realizing the 1D topological states. Here, we develop an extended version of the SSH model with multi-atomic bases of four, six and eight atoms and an imposed screw rotation symmetry. Furthermore, based on first-principles calculations, we demonstrate the realization of this model in transition metal monochalcogenide M6X6 (M = Mo and W; X = S, Se and Te) nanowires. The topological features of the doped M6X6 nanowires are confirmed with non-trivial edge modes and e/2 fractional charges, representative of the 1D non-trivial Zak phase. Our finding not only sheds new light on our fundamental understanding of 1D topological physics, but also significantly extends the scope of 1D topological materials that will attract immediate experimental interest, since isolated M6X6 nanowires have already been synthesized in experiments.

Isolation of Single-Wired Transition-Metal Monochalcogenides by Carbon Nanotubes

The successful isolation of single layers from two-dimensional (2D) van der Waals (vdW)-layered materials has opened new frontiers in condensed matter physics and materials science. Their discovery and unique properties laid the foundation for exploring 1D counterparts. However, the isolation of 1D vdW-wired materials has thus far remained a challenge, and effective techniques are demanded. Here we report the facile synthesis of isolated transition-metal monochalcogenide MoTe nanowires by using carbon nanotubes (CNTs) as molds. Individual nanowires are perfectly separated by CNTs with a minimal interaction, enabling detailed characterization of the single wires. Transmission electron microscopy revealed unusual torsional motion of MoTe nanowires inside CNTs. Confinement of 1D vdW-wired materials to the nanotest tubes might open up possibilities for exploring unprecedented properties of the nanowires and their potential applications such as electromechanical switching devices.

Unprecedented New Crystalline Forms of SnSe in Narrow to Medium Diameter Carbon Nanotubes

We report the observation of four unprecedented new crystalline forms of SnSe, obtained as a result of encapsulation in narrow to medium diameter single-walled carbon nanotubes. Aberration-corrected scanning transmission electron microscopy at 80 kV revealed linear, zigzag, helical (i.e., 2 × 1) atomic chains and a new form of encapsulated SnSe. This new form is apparently isostructural to free-standing MoS, MoSe, and WSe extreme nanowires etched from the corresponding monolayer dichalcogenides and also recently observed encapsulated MoTe. A structural model has been attained from annular dark-field (ADF) images. The experimental imaging agrees well with image simulations produced from models anticipated for the new structural forms.

In situ high temperature atomic level dynamics of large inversion domain formations in monolayer MoS2

Here we study the high-temperature formation and dynamics of large inversion domains (IDs) that form in monolayer MoS2 using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) with an in situ heating stage. We use temperatures above 700 °C to thermally activate rapid S vacancy migration and this leads to a formation mechanism of IDs that differs from the one at room temperature, where S vacancy migration is limited. We show that at high temperatures the formation of IDs occurs from intersected networks of long S vacancy line defects, whose strain fields are non-orthogonal and trigger large scale atomic reconstructions. Once formed, the IDs are influenced by the dynamic behaviour of nearby line defects and voids. With Mo and S atoms undergoing movement, the two types of ID grain boundaries can shift to allow further expansion of the ID area along the adjoining line defects. We reveal that IDs serve as metastable configurations between line defect rearrangements and eventual void formation under electron beam irradiation during heating. The formation of voids near to the IDs causes them to revert back to pristine lattice, which has the effect of restricting the ID domain size to a certain range (e.g. 3-5 nm in our observation) instead of continuously enlarging. This study provides insights into how the MoS2 IDs form and evolve at high temperature and can benefit the tailoring of electronic properties of two dimensional materials by structural manipulation.

Atomic structure of defects and dopants in 2D layered transition metal dichalcogenides

Transmission electron microscopy can directly image the detailed atomic structure of layered transition metal dichalcogenides, revealing defects and dopants.

In Situ Transmission Electron Microscopy Characterization and Manipulation of Two-Dimensional Layered Materials beyond Graphene

Two-dimensional (2D) ultra-thin materials beyond graphene with rich physical properties and unique layered structures are promising for applications in electronics, chemistry, energy, and bioscience, etc. The interaction mechanisms among the structures, chemical compositions and physical properties of 2D layered materials are critical for fundamental nanosciences and the practical fabrication of next-generation nanodevices. Transmission electron microscopy (TEM), with its high spatial resolution and versatile external fields, is undoubtedly a powerful tool for the static characterization and dynamic manipulation of nanomaterials and nanodevices at the atomic scale. The rapid development of thin-film and precision microelectromechanical systems (MEMS) techniques allows 2D layered materials and nanodevices to be probed and engineered inside TEM under external stimuli such as thermal, electrical, mechanical, liquid/gas environmental, optical, and magnetic fields at the nanoscale. Such advanced technologies leverage the traditional static TEM characterization into an in situ and interactive manipulation of 2D layered materials without sacrificing the resolution or the high vacuum chamber environment, facilitating exploration of the intrinsic structure-property relationship of 2D layered materials. In this Review, the dynamic properties tailored and observed by the most advanced and unprecedented in situ TEM technology are introduced. The challenges in spatial, time and energy resolution are discussed also.

Direct Imaging of the Onset of Electrical Conduction in Silver Nanowire Networks by Infrared Thermography: Evidence of Geometrical Quantized Percolation

Advancement in the science and technology of random metallic nanowire (MNW) networks is crucial for their appropriate integration in many applications including transparent electrodes for optoelectronics and transparent film heaters. We have recently highlighted the discontinuous activation of efficient percolating pathways (EPPs) for networks having densities slightly above the percolation threshold. Such networks exhibit abrupt drops of electrical resistance when thermal or electrical annealing is performed, which gives rise to a "geometrically quantized percolation". In this Letter, lock-in thermography (LiT) is used to provide visual evidence of geometrical quantized percolation: when low voltage is applied to the network, individual "illuminated pathways" can be detected, and new branches get highlighted as the voltage is incrementally increased. This experimental approach has allowed us to validate our original model and map the electrical and thermal distributions in silver nanowire (AgNW) networks. We also study the effects of electrode morphology and wire dimensions on quantized percolation. Furthermore, we demonstrate that the network failure at high temperature can also be governed by a quantized increase of the electrical resistance, which corresponds to the discontinuous destruction of individual pathways (antipercolation). More generally, we demonstrate that LiT is a promising tool for the detection of conductive subclusters as well as hot spots in AgNW networks.