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A new class of tetragonally symmetric 2D octagonal family of monolayers (o-MLs) has emerged recently and demands understanding at the fundamental level. o-MLs of metal nitride and carbide family (BN, AlN, GaN, GeC, SiC) along with C and BP are computationally designed and their stability and electronic structure are investigated. These binary o-MLs show mixed ionic and covalent bonding with the hybridized p states dominating the electronic structure around the Fermi level. Geometric and structural similarity of o-C and o-BN has been exploited to form patterned hybrid o-MLs ranging from metallic to insulating phases. Stacking of zigzag buckled o-MLs results in stable body centered tetragonal (bct)-bulk phase that is suitable for most materials from group IV, III-V and II-VI. Vertically cut chunks of o-BN and o-C bulk or stacking of o-rings, unlike rolling of hexagonal (h)-ML, provide a plausible way to form very thin o-nanotubes (o-NT). Confined and bulk structures formed with an octagonal motif are of fundamental importance to understand the underlying science and for technological applications.
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The transition-metal dichalcogenides (TMD) MoS2 and WS2 show remarkable electromechanical properties. Strain modifies the direct band gap into an indirect one, and substantial strain even induces an semiconductor-metal transition. Providing strain through mechanical contacts is difficult for TMD monolayers, but state-of-the-art for TMD nanotubes. We show using density-functional theory that similar electromechanical properties as in monolayer and bulk TMDs are found for large diameter TMD single- (SWNT) and multi-walled nanotubes (MWNTs). The semiconductor-metal transition occurs at elongations of 16 %. We show that Raman spectroscopy is an excellent tool to determine the strain of the nanotubes and hence monitor the progress of that nanoelectromechanical experiment in situ. TMD MWNTs show twice the electric conductance compared to SWNTs, and each wall of the MWNTs contributes to the conductance proportional to its diameter.
The ability to imprint a given material property to another through proximity effect in layered two-dimensional materials has opened the way to the creation of designer materials. Here, we use molecular-beam epitaxy (MBE) for a direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe$_2$) with the monolayer ferromagnetic chromium tribromide (CrBr$_3$). Using different characterization techniques and density-functional theory (DFT) calculations, we have confirmed that the CrBr$_3$ monolayer retains its ferromagnetic ordering with a magnetocrystalline anisotropy favoring an out-of-plane spin orientation. Low-temperature scanning tunneling microscopy (STM) measurements show a slight reduction of the superconducting gap of NbSe$_2$ and the formation of a vortex lattice on the CrBr$_3$ layer in experiments under an external magnetic field. Our results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.
We have discovered that the influence of the surrounding nanotubes in a bundle is similar to that of a liquid having surface tension equal to the surface energy of the nanotubes. This surprising behaviour is confirmed by the calculation of the self-collapse diameters of nanotubes in a bundle. Other systems, such as peapods, fullerites, are similarly treated, including the effect of the presence of a solvent. Finally, we have evaluated the strength and toughness of the nanotube bundle, with or without collapsed nanotubes, assuming a sliding failure.
We study the dependence of thermal conductivity of single walled nanotubes (SWNT) on chirality and isotope impurity by nonequilibrium molecular dynamics method with accurate potentials. It is found that, contrary to electronic conductivity, the thermal conductivity is insensitive to the chirality. The isotope impurity, however, can reduce the thermal conductivity up to 60% and change the temperature dependence behavior. We also study the dependence of thermal conductivity on tube length for tubes of different radius at different temperatures.
Excitons with binding energies of a few hundreds of meV control the optical properties of transition metal dichalcogenide monolayers. Knowledge of the fine structure of these excitons is therefore essential to understand the optoelectronic properties of these 2D materials. Here we measure the exciton fine structure of MoS2 and MoSe2 monolayers encapsulated in boron nitride by magneto-photoluminescence spectroscopy in magnetic fields up to 30 T. The experiments performed in transverse magnetic field reveal a brightening of the spin-forbidden dark excitons in MoS2 monolayer: we find that the dark excitons appear at 14 meV below the bright ones. Measurements performed in tilted magnetic field provide a conceivable description of the neutral exciton fine structure. The experimental results are in agreement with a model taking into account the effect of the exchange interaction on both the bright and dark exciton states as well as the interaction with the magnetic field.
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