Layer-by-Layer Dielectric Breakdown of Hexagonal Boron Nitride

Hexagonal boron nitride (BN) is widely used as a substrate and gate insulator for two-dimensional (2D) electronic devices. The studies on insulating properties and electrical reliability of BN itself, however, are quite limited. Here, we report a systematic investigation of the dielectric breakdown characteristics of BN using conductive atomic force microscopy. The electric field strength was found to be ∼12 MV/cm, which is comparable to that of conventional SiO₂ oxides because of the covalent bonding nature of BN. After the hard dielectric breakdown, the BN fractured like a flower into equilateral triangle fragments. However, when the applied voltage was terminated precisely in the middle of the dielectric breakdown, the formation of a hole that did not penetrate to the bottom metal electrode was clearly observed. Subsequent <i>I–V</i> measurements of the hole indicated that the BN layer remaining in the hole was still electrically inactive. On the basis of these observations, layer-by-layer breakdown was confirmed for BN with regard to both physical fracture and electrical breakdown. Moreover, statistical analysis of the breakdown voltages using a Weibull plot suggested the anisotropic formation of defects. These results are unique to layered materials and unlike the behavior observed for conventional 3D amorphous oxides

Crystalline Graphdiyne Nanosheets Produced at a Gas/Liquid or Liquid/Liquid Interface

Synthetic two-dimensional polymers, or bottom-up nanosheets, are ultrathin polymeric frameworks with in-plane periodicity. They can be synthesized in a direct, bottom-up fashion using atomic, ionic, or molecular components. However, few are based on carbon–carbon bond formation, which means that there is a potential new field of investigation into these fundamentally important chemical bonds. Here, we describe the bottom-up synthesis of all-carbon, π-conjugated graphdiyne nanosheets. A liquid/liquid interfacial protocol involves layering a dichloromethane solution of hexaethynylbenzene on an aqueous layer containing a copper catalyst at room temperature. A multilayer graphdiyne (thickness, 24 nm; domain size, >25 μm) emerges through a successive alkyne–alkyne homocoupling reaction at the interface. A gas/liquid interfacial synthesis is more successful. Sprinkling a very small amount of hexaethynylbenzene in a mixture of dichloromethane and toluene onto the surface of the aqueous phase at room temperature generated single-crystalline graphdiyne nanosheets, which feature regular hexagonal domains, a lower degree of oxygenation, and uniform thickness (3.0 nm) and lateral size (1.5 μm)

Crystalline Graphdiyne Nanosheets Produced at a Gas/Liquid or Liquid/Liquid Interface

Synthetic two-dimensional polymers, or bottom-up nanosheets, are ultrathin polymeric frameworks with in-plane periodicity. They can be synthesized in a direct, bottom-up fashion using atomic, ionic, or molecular components. However, few are based on carbon–carbon bond formation, which means that there is a potential new field of investigation into these fundamentally important chemical bonds. Here, we describe the bottom-up synthesis of all-carbon, π-conjugated graphdiyne nanosheets. A liquid/liquid interfacial protocol involves layering a dichloromethane solution of hexaethynylbenzene on an aqueous layer containing a copper catalyst at room temperature. A multilayer graphdiyne (thickness, 24 nm; domain size, >25 μm) emerges through a successive alkyne–alkyne homocoupling reaction at the interface. A gas/liquid interfacial synthesis is more successful. Sprinkling a very small amount of hexaethynylbenzene in a mixture of dichloromethane and toluene onto the surface of the aqueous phase at room temperature generated single-crystalline graphdiyne nanosheets, which feature regular hexagonal domains, a lower degree of oxygenation, and uniform thickness (3.0 nm) and lateral size (1.5 μm)

Expansion of the Graphdiyne Family: A Triphenylene-Cored Analogue

Graphdiyne (GDY) comprises an important class in functional covalent organic nanosheets based on carbon–carbon bond formation, and recent focus has collected in the expansion of its variations. Here we report on the synthesis of a GDY analogue, <b>TP-GDY</b>, which has triphenylene as the aromatic core. Our liquid/liquid interfacial synthesis for GDY (<i>J. Am. Chem. Soc.</i> <b>2017</b>, <i>139</i>, 3145) was modified for hexaethynyltriphenylene monomer to afford a <b>TP-GDY</b> film with a free-standing morphology, a smooth texture, a domain size of >1 mm, and a thickness of 220 nm. Resultant <b>TP-GDY</b> is characterized by series of microscopies, spectroscopies, and thermogravimetric and gas adsorption analyses

Hydrogen-Assisted Epitaxial Growth of Monolayer Tungsten Disulfide and Seamless Grain Stitching

Recently, research on transition metal dichalcogenides (TMDCs) has been accelerated by the development of large-scale synthesis based on chemical vapor deposition (CVD). However, in most cases, CVD-grown TMDC sheets are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs) which deteriorate the physical properties of the TMDC. Here, we demonstrate the epitaxial growth of monolayer tungsten disulfide (WS₂) on sapphire by introducing a high concentration of hydrogen during the CVD process. As opposed to the randomly oriented grains obtained in conventional growth, the presence of H₂ resulted in the formation of triangular WS₂ grains with the well-defined orientation determined by the underlying sapphire substrate. Photoluminescence of the aligned WS₂ grains was significantly suppressed compared to that of the randomly oriented grains, indicating a hydrogen-induced strong coupling between WS₂ and the sapphire surface that has been confirmed by density functional theory calculations. Scanning transmission electron microscope observations revealed that the epitaxially grown WS₂ has less structural defects and impurities. Furthermore, sparsely distributed unique dislocations were observed between merging aligned grains, indicating an effective stitching of the merged grains. This contrasts with the GBs that are observed between randomly oriented grains, which include a series of 8-, 7-, and alternating 7/5-membered rings along the GB. The GB structures were also found to have a strong impact on the chemical stability and carrier transport of merged WS₂ grains. Our work offers a novel method to grow high-quality TMDC sheets with much less structural defects, contributing to the future development of TMDC-based electronic and photonic applications