Hexagonal boron nitride (hBN) with a single boron isotope have many enhanced physical, thermal and optical properties compared to the most common hBN with the natural distribution of boron (19.9 at. % 10B and 80.1 at. % 11B). These property differences can significantly improve the device performance in applications, such as neutron detectors, nanoscale electronics, and optical components. In this study, a new method for the growth of large-scale, high-quality monoisotopic hBN single crystals, i.e., h10BN and h11BN, was developed. hBN single crystals were grown using a nickel–chromium solvent and pure boron and nitrogen sources at atmospheric pressure. The clear and colorless crystals have a maximum domain size of around 1 mm. Raman measurements demonstrate that the crystals produced with this method are pure hBN phase with low defect density, and the spectral peaks vary with the boron isotope concentrations. X-ray photoelectron spectroscopy spectra show that the B–N bond in h11BN is slightly stronger than that in h10BN. The ability to produce crystals in this manner opens the door to isotopically engineering the properties and performance of hBN devices.

The branched ionic liquids (ILs) 1-(iso-alkyl)-3-methylimidazolium bis­[(trifluoromethane)­sulfonyl]­amide ([(N – 2)­mC N‑1C1im]­[NTf2] with N = 3–7) were synthesized and their physicochemical properties characterized and compared with the properties of linear ILs 1-(n-alkyl)-3-methylimidazolium bis­[(trifluoromethane)­sulfonyl]­amide ([C N C1im]­[NTf2] with N = 3–7). For N = 4–7, the density of the branched IL [(N – 2)­mC N–1C1im]­[NTf2] is the same as that of its linear analogue [C N C1im]­[NTf2] within the standard uncertainty of the measurements. In the case of the N = 3 [1mC2C1im]­[NTf2]/[C3C1im]­[NTf2] pair, the density of the branched IL is 0.13% higher than that of the linear IL. For a branched/linear IL pair with a given N, the glass transition temperature T g, melting temperature T m, and viscosity η are higher for the branched IL than for the linear IL. [2mC3C1im]­[NTf2] is an exception in that its T m is lower than that of [C4C1im]­[NTf2]. Moreover, the viscosity of [2mC3C1im]­[NTf2] is anomalously higher than what would be predicted based on the trend of the other branched ILs. These trends in the viscosities of the linear and branched ILs are consistent with recent molecular dynamics simulations. Thermal gravimetric analysis indicates that linear ILs are thermally more stable than branched ILs. Pulsed-gradient spin–echo (PGSE) NMR diffusion measurements show that the self-diffusion coefficients of the ions vary inversely with the viscosities according to the Stokes–Einstein (SE) equation. The hydrodynamic radii of the cations and anions of linear ILs calculated from the SE equation however are consistently higher than those of the corresponding branched ILs.

A series of branched ionic liquids (ILs) based on the 1-(iso-alkyl)-3-methylimidazolium cation from 1-(1-methylethyl)-3-methylimidazolium bistriflimide to 1-(5-methylhexyl)-3-methylimidazolium bistriflimide and linear ILs based on the 1-(n-alkyl)-3-methylimidazolium cation from 1-propyl-3-methylimidazolium bistriflimide to 1-heptyl-3-methylimidazolum bistriflimide were recently synthesized and their physicochemical properties characterized. For the ILs with the same number of carbons in the alkyl chain, the branched IL was found to have the same density but higher viscosity than the linear one. In addition, the branched IL 1-(2-methylpropyl)-3-methylimidazolium bistriflimide ([2mC3C1Im][NTf2]) was found to have an abnormally high viscosity. Motivated by these experimental observations, the same ILs were studied using molecular dynamics (MD) simulations in the current work. The viscosities of each IL were calculated using the equilibrium MD method at 400 K and the nonequilibrium MD method at 298 K. The results agree with the experimental trend. The ion pair (IP) lifetime, spatial distribution function, and associated potential of mean force, cation size and shape, and interaction energy components were calculated from MD simulations. A quantitative correlation between the liquid structure and the viscosity was observed. Analysis shows that the higher viscosities in the branched ILs are due to the relatively more stable packing between the cations and anions indicated by the lower minima in the potential of mean force (PMF) surface. The abnormal viscosity of [2mC3C1Im][NTf2] was found to be the result of the specific side chain length and molecular structure.

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