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Mechanistic Insights into the Formation of Lithium Fluoride Nanotubes

Born-Oppenheimer molecular dynamics (BOMD) and periodic density functional theory (DFT) calculations have been applied for describing the mechanism of formation of lithium fluoride (LiF) nanotubes with cubic, hexagonal, octagonal, decagonal, dodecagonal, and tetradecagonal cross-sections. It has been shown that high energy structures, such as nanowires, nanorings, nanosheets, and nanopolyhedra are transient species for the formation of stable nanotubes. Unprecedented (LiF)_n clusters (n≤12) were also identified, some of them lying less than 10 kcal mol^-[1] above their respective global minima. Such findings indicate that stochastic synthetic techniques, such as laser ablation and chemical vapor deposition, should be combined with a template-driven procedure in order to generate the nanotubes with adequate efficiency. Apart from the stepwise growth of LiF units, the formation of nanotubes was also studied by rolling up a planar square sheet monolayer, which could be hypothetically produced from the exfoliation of the FCC crystal structure. It was shown that both pathways could lead to the formation of alkali halide nanotubes, a still unprecedented set of one-dimensional materials.

Alkali Halide Nanotubes: Structure and Stability

Accurate density functional theory (DFT) and coupled-cluster (CCSD) calculations on a series of (LiF) _n=2,36 neutral clusters suggest that nanotube structures with hexagonal and octagonal transversal cross sections show stability equal to or greater than that of the typical cubic form of large LiF crystals. The nanotube stability was further corroborated by quantum dynamic calculations at room temperature. The fact that stable nanotube structures were also found for other alkali halides (e.g., NaCl and KBr) suggests that this geometry may be widely implemented in material sciences.

Alkali halide clusters produced by fast ion impact

The most abundant geometries and relative stabilities of alkali halide clusters with a (XY)(n) (o) configuration (e.g., LiF, NaCl, KBr) are described. Five main series were obtained: linear, cyclic, cubic, arc strips and nanotubes. The stability analysis shows that higher members are likely to be formed from the lower member of the same series and/or from two building blocks (n = 1, 2). The energy analysis (D-plot) indicates that the most compact ones (e.g., cubic and nanotubes) present higher stability when compared to the linear, cyclic and arc strip structures; moreover, relative stability between the cubic and nanotube series varies with the cluster size.

Characterization of (NH3)(n=1-6)NH+ clusters produced by 252Cf fragments impact onto a NH3 condensed target

This paper reports the first characterization of the (NH(3))(n)NH+ cluster series produced by a 252Cf fission fragments (FF) impact onto a NH(3) ice target. The (NH(3))(n=1-6)NH+ members of this series have been analyzed theoretically and experimentally. Their ion desorption yields show an exponential dependence of the cluster population on its mass, presenting a relative higher abundance at n = 5. The results of DFT/B3LYP calculations show that two main series of ammonium clusters may be formed. Both series follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of the respective {NH(3)NH}+ and {NH(2)NH(2)}+ cores. The energy analysis (i.e., D-plot and stability analysis) shows that the calculated members of the (NH(3))(n-1){NH(2)NH(2)}+ series are more stable than those of the (NH(3))(n-1){NH(3)NH}+ series. The trend on the relative stability of the members of more stable series, (NH(3))(n-1){NH(2)NH(2)}+, shows excellent agreement with the experimental distribution of cluster abundances. In particular, the (NH(3))4{NH(2)NH(2)}+ structure is the most stable one, in agreement with the experiments.