Flexoelectricity in hexagonal boron nitride monolayers

Flexoelectricity and electronic properties of monolayer GaSe under shear strain gradient

This paper concerns the vibrational behavior of concentric double-walled and triple-walled carbon and boron nitride nanotubes using the finite element method. Armchair and zigzag nanotubes with different lengths and diameters are considered. Moreover, different boundary conditions are applied on the nanotubes. It is observed that in double-walled nanotubes, when the inner and outer layers are respectively from boron nitride and carbon, the frequencies are larger than those in the reverse arrangement. Investigating the effect of diameter on the first 10 natural frequencies of double-walled and triple-walled nanotubes showed that nanotubes with larger diameters possess smaller frequencies. The effect of diameter is more significant for higher modes. Finally, comparisons are made between the vibrational behavior of concentric carbon and boron nitride double-walled and triple-walled nanotubes. Considering the effect of vacancy defect on the vibrational characteristics of the nanotubes revealed that when all of the walls of the nanotubes are defective, the largest diminish occurs for the fundamental natural frequencies.

Energy Storage and conversion devices require an ion-exchange membrane to accomplish high transmission of charge-balancing ions and separation of anode and cathode electrolytes/gases from mixing. These critical functions of membranes play a pivotal role in ensuring device optimum performance and higher efficiency. Most conventional membranes suffer huge ionic and molecular species cross-permeation resulting in low energy efficiency and material degradation. In this work, hydrogen permeability and proton transmission through membrane electrode assemblies (MEAs) that contain a monolayer of hexagonal boron nitride, single-layer and bi-layer graphene were investigated in a gas-phase small-scale cell and a liquid cell. We found that the hydrogen crossover flux through MEAs with 2D materials was inhibited by at least a factor of 5 as compared to the one without. Single-layer graphene and boron nitride enabled high proton transmission, but bi-layer graphene inhibited proton conduction. Defect visualization of 2D materials by chemical treatment with a low ferric chloride concentration followed by imaging using a digital microscope revealed few atomic-scale defects in graphene. These findings suggest that a monolayer of 2D material may provide good selectivity for energy conversion and storage devices by blocking species crossover while allowing high proton transmission.

Energy storage and conversion devices require an ion-exchange membrane with high transmission of charge-balancing ions and separation of anode and cathode electrolytes/gases. This ensures optimum device performance. Most conventional membranes suffer huge cross-permeation resulting in low energy efficiency and material degradation. This work investigated hydrogen permeability and proton transmission through membrane electrode assemblies (MEAs) containing a monolayer of hexagonal boron nitride and single-layer and bi-layer graphene in a gas-phase small-scale cell and a liquid cell. We found that the hydrogen crossover flux through MEAs with 2D materials was inhibited by at least a factor of 5 compared to the one without. Single-layer graphene and boron nitride enabled high proton transmission, but bi-layer graphene inhibited proton conduction. Defect visualization of 2D materials revealed few atomic-scale defects in graphene. These findings suggest that a monolayer of 2D material may provide good selectivity for energy conversion and storage devices by blocking species crossover while allowing high proton transmission.

We theoretically study physical properties of the most promising color center candidates for the recently observed single-photon emissions in hexagonal boron nitride (h-BN) monolayers. Through our group theory analysis combined with density functional theory (DFT) calculations we provide several pieces of evidence that the electronic properties of the color centers match the characters of the experimentally observed emitters. We calculate the symmetry-adapted multi-electron wavefunctions of the defects using group theory methods and analyze the spin-orbit and spin-spin interactions in detail. We also identify the radiative and non-radiative transition channels for each color center. An advanced ab-initio DFT method is then used to compute energy levels of the color centers and their zero-phonon-line (ZPL) emissions. The computed ZPLs, the profile of excitation and emission dipole polarizations, and the competing relaxation processes are discussed and matched with the observed emission lines. By providing evidence for the relation between single-photon emitters and local defects in h-BN, this work provides the first steps towards harnessing quantum dynamics of these color centers.

We combine first-principles molecular-dynamics simulations with high-resolution transmission electron microscopy experiments to draw a detailed microscopic picture of irradiation effects in hexagonal boron nitride ($h$-BN) monolayers. We determine the displacement threshold energies for boron and nitrogen atoms in $h$-BN, which differ significantly from the tight-binding estimates found in the literature and remove ambiguity from the interpretation of the experimental results. We further develop a kinetic Monte Carlo model which allows to extend the simulations to macroscopic time scales and make a direct comparison between theory and experiments. Our results provide a comprehensive picture of the response of $h$-BN nanostructures to electron irradiation.

Annealing of a monolayer of hexagonal boron nitride destroyed by Xe ion irradiation gives rise to rich structural phenomena investigated here through a combination of scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron spectroscopy, and density functional theory calculations. We find selective pinning of vacancy clusters at a single specific location within the moiré formed by hexagonal boron nitride (h-BN) and the Ir substrate, crystalline Xe at room temperature of monolayer and bilayer thickness sealed inside h-BN blisters, standalone blisters only bound to the metal at temperatures where boron nitride on Ir(111) decomposes, and finally a pronounced threefold symmetry of all morphological features due to the preferential formation of boron-terminated zigzag edges that firmly bind to the substrate. The investigations give clear insight into the relevance of the substrate for the damage creation and annealing in a two-dimensional layer material.

The fascinating two-dimensional (2D) materials are being unconsciously applied in various fields from science to engineering, which is benefited from the glamorous physical and chemical properties of mechanics, optics, electronics, and magnetism. The representative 2D thermoelectric/piezoelectric materials can directly convert thermal/mechanical energy into electrical energy, which can resolve the energy issues and avoid further environmental deterioration. The thermoelectric or piezoelectric properties of various 2D materials, such as graphene, hexagonal boron nitride, black phosphorus, transition metal dichalcogenides (TMDs), arsenene, metal carbides and nitrides (MXenes), and so on, have been investigated in detail. Although tremendous progress has been achieved in the past few years, these properties still need to be improved for their practical application by designing new 2D materials, strain engineering, chemical functionalization, etc. In addition to this, in 2D materials, there are many other novel physical properties, such as magnetism, topology, valley, and so on. The combination of thermoelectricity/piezoelectricity with other unique properties may lead to novel device applications or scientific breakthroughs in new physics. Overall, the emergence of 2D thermoelectric and piezoelectric materials has expanded energy conversion research dramatically. By combing this new device concept with the novel 2D materials, original devices should have potential applications in energy harvesting.

Due to their outstanding electronic and physical properties, two-dimensional (2D) materials have attracted much interest for the fabrication of solid-state microelectronic devices. Among all methods to synthesize 2D materials, chemical vapor deposition (CVD) is the most attractive in the field of solid-state microelectronics because it can produce high quality 2D material in a scalable manner. However, the high temperatures (>900 °C) required during the CVD growth of the 2D materials impede their direct synthesis on metal-coated wafers due to prohibitive metal diffusion and de-wetting. This makes necessary carrying out the 2D materials CVD growth independently on metallic foils, and transfer them on the wafers using polymer scaffolds. However, this process is slower, more expensive, and can lead to abundant contamination and cracks in the 2D material. Here we present a facile method to allow the direct growth of multilayer hexagonal boron nitride (h-BN) on Ni-coated Si wafers, which consists on placing a protective cover 30 μm above the Ni surface. The resulting h-BN stacks have been used to fabricate Au/Ti/h-BN/Ni memristors with low cycle-to-cycle variability. This work contributes to the integration of 2D materials in solid-state micro- and nano-electronic technologies.

A systematic review on 2D materials for volatile organic compound sensing

Photonic platforms with excellent nonlinear optical characteristics are very important to improve the devices' performance parameters such as integration, modulation speeds and working bandwidths for all-optical signal processing. The traditional processing technology of photonic platforms based on silicon, silicon nitride and silicon oxide is mature, but the nonlinear function of these optical platforms is limited due to the characteristics of materials; Although two-dimensional (2D) materials possess excellent nonlinear optical properties, their nonlinear potentials cannot be fully utilized because of their atomic layer thickness. Integrating 2D materials with mature photonic platforms can significantly improve the interaction between light and matter, give full play to the potentials of 2D materials in the field of nonlinear optics, and improve the nonlinear optical performances of the integrated platforms on the basis of fully utilizing the mature processing technology of the photonic platforms. Based on the above ideas, starting from the basic principle of nonlinear optics (Section 2), this review combs the research progress of various nonlinear photonic platforms (resonators, metasurfaces, optical fibers, on-chip waveguides, etc.) heterogeneously integrated with 2D materials, realized by traditional transfer methods (Section 3) and emerging direct-growth methods (Section 4) in recent years, and the introduction is divided into second-order and third-order nonlinearity. Comparing with the transfer methods, the advantages of using direct-growth methods to realize the heterogeneous integration of 2D materials and photonic platforms for the study of nonlinear optics are expounded, and the technical difficulties to be overcome in preparing the actual devices are also pointed. In the future, we can try to grow 2D materials directly onto the surfaces of various cavities to study the enhancement of second-order nonlinearity; we can also try to grow 2D materials directly onto the on-chip waveguides or microrings to study the enhancement of third-order nonlinearity. Generally speaking, the research on integrated nonlinearity by directly growing 2D materials onto various photonic structures has aroused great interest of researchers in this field. As time goes on, breakthrough progress will be made in this field, and technical problems such as continuous growth of high-quality 2D materials onto photonic structures and wafer-level large-scale preparation will be broken through, further improving the performance parameters of chips and laying a good foundation for optical communication, signal processing, optical sensing, all-optical computing, quantum technology and so on.

Most optoelectronic characteristics of two-dimensional (2D) materials are associated with excitonic effects. Excitonic effects in 2D material have been intensively investigated, and various efforts to engineer exciton behavior in 2D materials have been reported for advanced nanophotonic and optoelectronic applications. Excitons in 2D semiconductors can be controlled by external stimuli, including mechanical, electrical, thermal, and magnetic stimuli. Mechanical stimuli applied to a 2D material can generate uniform or non-uniform deformation and strain gradient in the 2D lattice, which creates a strain-induced bandgap energy gradient in the 2D material. In an inhomogeneous bandgap energy gradient generated by a non-uniform strain gradient, excitons drift across the energy gradient. Exciton engineering in deformed 2D materials aims to control exciton movement by mechanical strain. In this snapshot review, we focus on exciton engineering in a mechanically deformed 2D material and their potential towards advanced optoelectronic and photonic applications.

Physicochemical and biological characterization of single-walled and double-walled carbon nanotubes in biological media

The scalable preparation of high‐quality and low‐cost two‐dimensional (2D) materials is critical to achieving their potential applications in various fields. Chemical vapor deposition (CVD) method is considered the most promising method for producing ultrathin 2D materials and has continued to develop in recent years. First‐principles calculations have provided important theoretical guidance for the CVD synthesis of 2D materials, and have played an increasingly important role in the field of material synthesis in recent years. In this review, we present recent advances in the growth mechanism of 2D materials, focusing on the theoretical research progress of four typical 2D materials: graphene, hexagonal boron nitride (hBN), transition metal dichalcogenide (TMDC), and phosphorene. Several aspects of the growth process are discussed in detail, including the decomposition of precursors, nucleation, growth kinetics, domain shape, and epitaxial and alignment of 2D crystals. Based on the understanding of these atomic‐scale growth processes, strategies toward the wafer‐scale growth of continuous and homogeneous 2D thin films are proposed and confirmed by experiments. In the final section, we summarize future challenges and opportunities in the computational studies of the growth mechanism of 2D materials.This article is categorized under: Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics

Two-dimensional (2D) materials are considered for numerous applications in microelectronics, although several challenges remain when integrating them into functional devices. Weak adhesion is one of them, caused by their chemical inertness. Quantifying the adhesion of 2D materials on three-dimensional surfaces is, therefore, an essential step toward reliable 2D device integration. To this end, button shear testing is proposed and demonstrated as a method for evaluating the adhesion of 2D materials with the examples of graphene, hexagonal boron nitride (hBN), molybdenum disulfide, and tungsten diselenide on silicon dioxide and silicon nitride substrates. We propose a fabrication process flow for polymer buttons on the 2D materials and establish suitable button dimensions and testing shear speeds. We show with our quantitative data that low substrate roughness and oxygen plasma treatments on the substrates before 2D material transfer result in higher shear strengths. Thermal annealing increases the adhesion of hBN on silicon dioxide and correlates with the thermal interface resistance between these materials. This establishes button shear testing as a reliable and repeatable method for quantifying the adhesion of 2D materials.