Ripples, strain, and misfit dislocations: structure of graphene-boron nitride superlattice interfaces

Analysis of Strain and Defects in Tellurium-WSe2 Moiré Heterostructures Using Scanning Nanodiffraction

In recent years, there has been an increasing focus on 2D nongraphene materials that range from insulators to semiconductors to metals. As a single-elemental van der Waals semiconductor, tellurium (Te) has captivating anisotropic physical properties. Recent work demonstrated growth of ultrathin Te on WSe2 with the atomic chains of Te aligned with the armchair directions of the substrate using physical vapor deposition (PVD). In this system, a moiré superlattice is formed where micrometer-scale Te flakes sit on top of the continuous WSe2 film. Here, we determined the precise orientation of the Te flakes with respect to the substrate and detailed structure of the resulting moiré lattice by combining electron microscopy with image simulations. We directly visualized the moiré lattice using center of mass-differential phase contrast (CoM-DPC). We also investigated the local strain within the Te/WSe2 layered materials using scanning nanodiffraction techniques. There is a significant tensile strain at the edges of flakes along the direction perpendicular to the Te chain direction, which is an indication of the preferred orientation for the growth of Te on WSe2. In addition, we observed local strain relaxation regions within the Te film, specifically attributed to misfit dislocations, which we characterize as having a screw-like nature. The detailed structural analysis gives insight into the growth mechanisms and strain relaxation in this moiré heterostructure.

Non-Equilibrium Thermodynamics of Heat Transport in Superlattices, Graded Systems, and Thermal Metamaterials with Defects

In this review, we discuss a nonequilibrium thermodynamic theory for heat transport in superlattices, graded systems, and thermal metamaterials with defects. The aim is to provide researchers in nonequilibrium thermodynamics as well as material scientists with a framework to consider in a systematic way several nonequilibrium questions about current developments, which are fostering new aims in heat transport, and the techniques for achieving them, for instance, defect engineering, dislocation engineering, stress engineering, phonon engineering, and nanoengineering. We also suggest some new applications in the particular case of mobile defects.

Transparent and high-performance electromagnetic interference shielding composite film based on single-crystal graphene/hexagonal boron nitride heterostructure

The multiple requirements of optical transmittance, high shielding effectiveness, and long-term stability bring considerable challenge to electromagnetic interference (EMI) shielding in the fields of visualization windows, transparent optoelectronic devices, and aerospace equipment. To this end, attempts were hereby made, and based on high-quality single crystal graphene (SCG)/hexagonal boron nitride (h-BN) heterostructure, transparent EMI shielding films with weak secondary reflection, nanoscale ultra-thin thickness and long-term stability were finally realized by a composite structure. In this novel structure, SCG was adopted as the absorption layer, while sliver nanowires (Ag NWs) film acted as the reflection layer. These two layers were placed on different sides of the quartz to form a cavity, which achieved the dual coupling effect, so that the electromagnetic wave was reflected multiple times to form more absorption loss. Among the absorption dominant shielding films, the composite structure in this work demonstrated stronger shielding effectiveness of 28.76 dB with a higher light transmittance of 80.6%. In addition, under the protection of the outermost h-BN layer, the decline range of the shielding performance of the shielding film was extensively reduced after 30 days of exposure to air and maintained long-term stability. Overall, this study provides an outstanding EMI shielding material with great potential for practical applications in electronic devices protection.

Strain Engineering of Low-Dimensional Materials for Emerging Quantum Phenomena and Functionalities

Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron-electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.

A theoretical study of wrinkle propagation in graphene with flower-like grain boundaries

This study investigated dynamic surface wrinkle propagation across a series of flower-like rotational grain boundaries (GBs) in graphene using theoretical solutions and atomistic simulations. It was found that there was significantly less out-of-plane displacement of dynamic wrinkles when curvature of rotational GBs was reduced, which can be explained by a defect shielding effect of flower-like GBs. Potential energy evolved via different modes for pristine graphene and graphene with various GBs. With external excitation, the distinctly different patterns of wrinkle propagation in graphene with various GBs demonstrated how dynamic wrinkling can reveal defects. These results can provide a theoretical basis for guiding the design and implementation of graphene-based nano-mechanical devices such as protectors and detectors.

Research on the correlation of mechanical properties of BN-graphene-BN/BN vertically-stacked nanostructures in the presence of interlayer sp3 bonds and nanopores with temperature

In this study, we investigate the coupling of an internal field (defect field-sp³ bonds and nanopores) and an external field (strain and temperature). Simultaneously, we provide a design idea of hybrid materials. The mechanical properties of hybrid materials under the condition of internal and external field coupling were studied. When nanopores and sp³ bonds are considered simultaneously, we found that internal (sp³ bonds and defects) and external field (temperature and strain fields) have a negative chain reaction on the mechanical properties of BN-graphene-BN/BN vertically-stacked nanostructures, and the negative chain reaction will gradually increase with the change in parameters (such as the increase in temperature). The sp³ bonds can be regarded as a special defect, which will increase the initial strain of the system. In addition, the mechanical properties of the nanostructure, containing square nanopores in the boron nitride region are most sensitive to temperature change, relative to the nanopore in the other two regions. Atoms (around square nanopores) are more likely to overcome the binding energy and lose stability from the inherent equilibrium position, relative to that of circular nanopores.

Superlattices based on van der Waals 2D materials

Two-dimensional (2D) materials exhibit a number of improved mechanical, optical, and electronic properties compared to their bulk counterparts. The absence of dangling bonds in the cleaved surfaces of these materials allows combining different 2D materials into van der Waals heterostructures to fabricate p-n junctions, photodetectors, and 2D-2D ohmic contacts that show unexpected performances. These intriguing results are regularly summarized in comprehensive reviews. A strategy to tailor their properties even further and to observe novel quantum phenomena consists in the fabrication of superlattices whose unit cell is formed either by two dissimilar 2D materials or by a 2D material subjected to a periodic perturbation, each component contributing with different characteristics. Furthermore, in a 2D material-based superlattice, the interlayer interaction between the layers mediated by van der Waals forces constitutes a key parameter to tune the global properties of the superlattice. The above-mentioned factors reflect the potential to devise countless combinations of van der Waals 2D material-based superlattices. In the present feature article, we explain in detail the state-of-the-art of 2D material-based superlattices and describe the different methods to fabricate them, classified as vertical stacking, intercalation with atoms or molecules, moiré patterning, strain engineering and lithographic design. We also aim to highlight some of the specific applications of each type of superlattices.

Low temperature epitaxy of tungsten–telluride heterostructure films

Single-crystal like WTe₂ films are grown by exploiting van der Waals epitaxy at low temperatures, using pulsed laser deposition.

Strain Mapping of Two-Dimensional Heterostructures with Subpicometer Precision

Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable strain, dislocations, or ripples, which can strongly affect their mechanical, optical, and electronic properties. We have developed an approach to map 2D heterojunction lattice and strain profiles with subpicometer precision and the ability to identify dislocations and out-of-plane ripples. We collected diffraction patterns from a focused electron beam for each real-space scan position with a high-speed, high dynamic range, momentum-resolved detector-the electron microscope pixel array detector (EMPAD). The resulting four-dimensional (4D) phase space data sets contain the full spatially resolved lattice information on the sample. By using this technique on tungsten disulfide (WS₂) and tungsten diselenide (WSe₂) lateral heterostructures, we have mapped lattice distortions with 0.3 pm precision across multimicron fields of view and simultaneously observed the dislocations and ripples responsible for strain relaxation in 2D laterally epitaxial structures.

Three-dimensional Architecture Enabled by Strained Two-dimensional Material Heterojunction

Engineering the structure of materials endows them with novel physical properties across a wide range of length scales. With high in-plane stiffness and strength, but low flexural rigidity, two-dimensional (2D) materials are excellent building blocks for nanostructure engineering. They can be easily bent and folded to build three-dimensional (3D) architectures. Taking advantage of the large lattice mismatch between the constituents, we demonstrate a 3D heterogeneous architecture combining a basal Bi₂Se₃ nanoplate and wavelike Bi₂Te₃ edges buckling up and down forming periodic ripples. Unlike 2D heterostructures directly grown on substrates, the solution-based synthesis allows the heterostructures to be free from substrate influence during the formation process. The balance between bending and in-plane strain energies gives rise to controllable rippling of the material. Our experimental results show clear evidence that the wavelengths and amplitudes of the ripples are dependent on both the widths and thicknesses of the rippled material, matching well with continuum mechanics analysis. The rippled Bi₂Se₃/Bi₂Te₃ heterojunction broadens the horizon for the application of 2D materials heterojunction and the design and fabrication of 3D architectures based on them, which could provide a platform to enable nanoscale structure generation and associated photonic/electronic properties manipulation for optoelectronic and electromechanic applications.

Conjugated π electron engineering of generalized stacking fault in graphene and h-BN

Generalized-stacking-fault energy (GSFE) serves as an important metric that prescribes dislocation behaviors in materials. In this paper, utilizing first-principle calculations and chemical bonding analysis, we studied the behaviors of generalized stacking fault in graphene and h-BN. It has been shown that the π bond formation plays a critical role in the existence of metastable stacking fault (MSF) in graphene and h-BN lattice along certain slip directions. Chemical functionalization was then proposed as an effective means to engineer the π bond, and subsequently MSF along dislocation slips within graphene and h-BN. Taking hydrogenation as a representative functionalization method, we demonstrated that, with the preferential adsorption of hydrogen along the slip line, π electrons along the slip would be saturated by adsorbed hydrogen atoms, leading to the moderation or elimination of MSF. Our study elucidates the atomic mechanism of MSF formation in graphene-like materials, and more generally, provides important insights towards predictive tuning of mechanic properties in two-dimensional nanomaterials.

Strain engineering of van der Waals heterostructures

Modifying the strain state of solids allows control over a plethora of functional properties. The weak interlayer bonding in van der Waals (vdWaals) materials such as graphene, hBN, MoS₂, and Bi₂Te₃ might seem to exclude strain engineering, since strain would immediately relax at the vdWaals interfaces. Here we present direct observations of the contrary by showing growth of vdWaals heterostructures with persistent in-plane strains up to 5% and we show that strain relaxation follows a not yet reported process distinctly different from strain relaxation in three-dimensionally bonded (3D) materials. For this, 2D bonded Bi₂Te₃-Sb₂Te₃ and 2D/3D bonded Bi₂Te₃-GeTe multilayered films are grown using Pulsed Laser Deposition (PLD) and their structure is monitored in situ using Reflective High Energy Electron Diffraction (RHEED) and post situ analysis is performed using Transmission Electron Microscopy (TEM). Strain relaxation is modeled and found to solely depend on the layer being grown and its initial strain. This insight demonstrates that strain engineering of 2D bonded heterostructures obeys different rules than hold for epitaxial 3D materials and opens the door to precise tuning of the strain state of the individual layers to optimize functional performance of vdWaals heterostructures.

Uncoiling of helical boron nitride-graphene nanoribbons in a single-walled carbon nanotube

Molecular dynamics simulation has been employed to study the encapsulation of boron nitride-graphene nanoribbons (BNCNRs) in a single-walled carbon nanotube (SWNT). The simulation results show that a helical BNCNR with large curvature can uncoil repeatedly and spontaneously in the SWNT, like the unwinding of the DNA in the nucleus. The uncoiling of the BNCNRs is accompanied by a system energy exchange between non-bonding energy and elastic potential energy due to the competition between the induction of graphene nanoribbon (GNR) segments and the resistance of boron nitride nanoribbon (BNNR) segments. The electronic transmission capacity of the BNCNR changes with the helical angle of the BNCNR, suggesting a changing electrical signal in the uncoiling and spiraling process. This study provides the opportunity to understand the encapsulation of the BNCNR in the SWNT in detail.

Topological Defects at the Graphene/h-BN interface Abnormally Enhance Its Thermal Conductance

Low thermal conductance across interface is often the limiting factor in managing heat in many advanced device applications. The most commonly used approach to enhance the thermal conductance is to reduce/eliminate the interfacial structural defects. Using a graphene/h-BN (Gr/h-BN) interface, we show surprisingly that topological defects are able to enhance the thermal conductance across the interface. It is found that the phonon transmission across the Gr/h-BN interface with 5|7 defects is higher than that of the pristine interface, which is in strong contrast to the common notion that interface defects promote phonon scattering. By analyzing the strain distribution and phonon vibrational spectra, we find that this abnormal enhancement in interfacial thermal conductance originates from the localization of the stress fields arising from misfit dislocations and their out-of-plane deformations at the interface. In the presence of the defects, the overall mismatch strain is reduced. In addition, the out-of-plane deformations screen the long-ranged dislocation strain fields, resulting in the stress fields to be localized only at the cores of the defects. This abnormal mechanism provides a new dimension to enhance the interfacial thermal conductance in two-dimensional heterostructures.