Evidence for Active Atomic Defects in Monolayer Hexagonal Boron Nitride: A New Mechanism of Plasticity in Two-Dimensional Materials

We report the formation and motion of 4|8 (square-octagon) defects in monolayer hexagonal boron nitride (h-BN). The 4|8 defects, involving less-favorable B–B and N–N bonds, are mobile within the monolayer at high sample temperature (∼1000 K) under electron beam irradiation. Gliding of one or two atomic rows along the armchair direction is suggested to be the origin of the defect motion. This represents a completely new mechanism of plasticity in two-dimensional materials

Synthesis and Atomic Characterization of a Ti₂O₃ Nanosheet

Titanium oxide nanosheets have been attracting much attention owing to their photocatalytic property. Here, we synthesized a Ti₂O₃ nanosheet by the reduction of a titania nanosheet (Ti_0.87O₂) that was one or two atoms in thickness. The atomic structure of the Ti₂O₃ nanosheet was quantitatively revealed by electron diffraction analysis, electron energy-loss spectroscopy, and high-resolution transmission electron microscopy (TEM). A titania nanosheet (Ti_0.87O₂) consisting of edge-shared TiO₆ octahedra was transformed to a Ti₂O₃ nanosheet consisting of face-shared octahedra by electron beam irradiation. This represents a stable crystal phase of titania nanosheets like the Magneli phase in oxygen-deficient environments. The atomic arrangement of the Ti₂O₃ nanosheet was directly observed by newly developed aberration-corrected TEM

Direct Observation of Band Structure Modifications in Nanocrystals of CsPbBr₃ Perovskite

We investigate the variation of the bandgap energy of single quantum dots of CsPbBr₃ inorganic halide perovskite as a function of size and shape and upon embedding within an ensemble. For that purpose, we make use of valence-loss electron spectroscopy with <i>Z</i>-contrast annular dark-field (ADF) imaging in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope. In the experiment, energy absorption is directly mapped onto individual quantum dots, whose dimensions and location are simultaneously measured to the highest precision. In that way, we establish an intimate relation between quantum dot size and even shape and its bandgap energy on a single object level. We explicitly follow the bandgap increase in smaller quantum dots due to quantum confinement and demonstrate that it is predominantly governed by the smallest of the three edges of the cuboidal perovskite dot. We also show the presence of an effective coupling between proximal dots in an ensemble, leading to band structure modification. These unique insights are directly relevant to the development of custom-designed quantum structures and solids which will be realized by purposeful assemblage of individually characterized and selected quantum dots, serving as building blocks

Correction to “Synthesis and Atomic Characterization of a Ti₂O₃ Nanosheet”

Correction to “Synthesis and Atomic Characterization of a Ti₂O₃ Nanosheet

Unexpected Huge Dimerization Ratio in One-Dimensional Carbon Atomic Chains

Peierls theory predicted atomic distortion in one-dimensional (1D) crystal due to its intrinsic instability in 1930. Free-standing carbon atomic chains created in situ in transmission electron microscope (TEM)− are an ideal example to experimentally observe the dimerization behavior of carbon atomic chain within a finite length. We report here a surprisingly huge distortion found in the free-standing carbon atomic chains at 773 K, which is 10 times larger than the value expected in the system. Such an abnormally distorted phase only dominates at the elevated temperatures, while two distinct phases, distorted and undistorted, coexist at lower or ambient temperatures. Atom-by-atom spectroscopy indeed shows considerable variations in the carbon 1s spectra at each atomic site but commonly observes a slightly downshifted π* peak, which proves its sp¹ bonding feature. These results suggest that the simple model, relaxed and straight, is not fully adequate to describe the realistic 1D structure, which is extremely sensitive to perturbations such as external force or boundary conditions

Unexpected Huge Dimerization Ratio in One-Dimensional Carbon Atomic Chains

Peierls theory predicted atomic distortion in one-dimensional (1D) crystal due to its intrinsic instability in 1930. Free-standing carbon atomic chains created in situ in transmission electron microscope (TEM)− are an ideal example to experimentally observe the dimerization behavior of carbon atomic chain within a finite length. We report here a surprisingly huge distortion found in the free-standing carbon atomic chains at 773 K, which is 10 times larger than the value expected in the system. Such an abnormally distorted phase only dominates at the elevated temperatures, while two distinct phases, distorted and undistorted, coexist at lower or ambient temperatures. Atom-by-atom spectroscopy indeed shows considerable variations in the carbon 1s spectra at each atomic site but commonly observes a slightly downshifted π* peak, which proves its sp¹ bonding feature. These results suggest that the simple model, relaxed and straight, is not fully adequate to describe the realistic 1D structure, which is extremely sensitive to perturbations such as external force or boundary conditions

Extraordinary Interfacial Stitching between Single All-Inorganic Perovskite Nanocrystals

All-inorganic cesium lead halide perovskite nanocrystals are extensively studied because of their outstanding optoelectronic properties. Being of a cubic shape and typically featuring a narrow size distribution, CsPbX₃ (X = Cl, Br, and I) nanocrystals are the ideal starting material for the development of homogeneous thin films as required for photovoltaic and optoelectronic applications. Recent experiments reveal spontaneous merging of drop-casted CsPbBr₃ nanocrystals, which is promoted by humidity and mild-temperature treatments and arrested by electron beam irradiation. Here, we make use of atom-resolved annular dark-field imaging microscopy and valence electron energy loss spectroscopy in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope to investigate the aggregation between individual nanocrystals at the atomic level. We show that the merging process preserves the elemental composition and electronic structure of CsPbBr₃ and takes place between nanocrystals of different sizes and orientations. In particular, we reveal seamless stitching for aligned nanocrystals, similar to that reported in the past for graphene flakes. Because the crystallographic alignment occurs naturally in drop-casted layers of CsPbX₃ nanocrystals, our findings constitute the essential first step toward the development of large-area nanosheets with band gap energies predesigned by the nanocrystal choicethe gateway to large-scale photovoltaic applications of inorganic perovskites

Auto-optimizing Hydrogen Evolution Catalytic Activity of ReS₂ through Intrinsic Charge Engineering

Optimizing active electronic states responding to catalysis is of paramount importance for developing high-activity catalysts because thermodynamics itself may not favor forming an optimal electronic state. Setting the monolayer transition metal dichalcogenide (TMD) ReS₂ as a model for the hydrogen evolution reaction (HER), we uncover that intrinsic charge engineering has an auto-optimizing effect on enhancing catalytic activity through regulating active electronic states. The experimental and theoretical results show that intrinsic charge compensation from S to Re–Re bonds could manipulate the active electronic states, allowing hydrogen to absorb the active sites neither strongly nor weakly. Two types of S sites exhibit the optimal hydrogen adsorption free energies (Δ<i>G</i>_H*) of 0.016 and 0.061 eV, which are the closest to zero corresponding to the highest HER activity. This auto-optimization via charge engineering is further demonstrated by higher turnover frequency per sulfur atom of 1–10 s^–1 and lower overpotential of −147 mV at 10 mA cm^–2 than those of other TMDs through multiscale activation and optimization. This work opens an avenue in designing extensive active catalysts through intrinsic charge engineering strategy

Controlled Synthesis of Atomically Thin 1T-TaS₂ for Tunable Charge Density Wave Phase Transitions

The charge density wave (CDW) in two-dimensional (2D) materials is attracting substantial interest because of its magnificent many-body collective phenomena. Various CDW phases have been observed in several 2D materials before they reach the phase of superconductivity. However, to date, the atomically thin CDW materials were mainly fabricated by mechanically exfoliating from their bulk counterparts, which leads to low production yield and small sample sizes. Here, we report the controlled synthesis of atomically thin 1T-TaS₂, a typical CDW material, by a chemical vapor deposition (CVD) method. The high quality of as-grown 1T-TaS₂ has been confirmed by complementary characterization technologies. Moreover, the thickness-dependent CDW phase transitions have been revealed in these ultrathin flakes by temperature-dependent Raman spectra. This work opens up a new window for the large-scale synthesis of ultrathin CDW materials and sheds light on the fabrication of next-generation electronic devices

Scalable van der Waals Heterojunctions for High-Performance Photodetectors

Atomically thin two-dimensional (2D) materials have attracted increasing attention for optoelectronic applications in view of their compact, ultrathin, flexible, and superior photosensing characteristics. Yet, scalable growth of 2D heterostructures and the fabrication of integrable optoelectronic devices remain unaddressed. Here, we show a scalable formation of 2D stacks and the fabrication of phototransistor arrays, with each photosensing element made of a graphene–WS₂ vertical heterojunction and individually addressable by a local top gate. The constituent layers in the heterojunction are grown using chemical vapor deposition in combination with sulfurization, providing a clean junction interface and processing scalability. The aluminum top gate possesses a self-limiting oxide around the gate structure, allowing for a self-aligned deposition of drain/source contacts to reduce the access (ungated) channel regions and to boost the device performance. The generated photocurrent, inherently restricted by the limited optical absorption cross section of 2D materials, can be enhanced by 2 orders of magnitude by top gating. The resulting photoresponsivity can reach 4.0 A/W under an illumination power density of 0.5 mW/cm², and the dark current can be minimized to few picoamperes, yielding a low noise-equivalent power of 2.5 × 10^–16 W/Hz^1/2. Tailoring 2D heterostacks as well as the device architecture moves the applications of 2D-based optoelectronic devices one big step forward