Alkali Halide Nanocrystals Research Articles

Graphitic cage transformation by electron-beam-induced catalysis with alkali-halide nanocrystals

We found that alkali-halide nanocrystals, such as KCl and NaCl, have strong catalytic capability to form graphitic carbon cages from amorphous carbon shells under electron beam irradiation. In addition to the electron beam irradiation strongly inducing the decomposition of alkali-halide nanocrystals, graphene fragments were formed and linked together to form the final product of thin graphitic carbon cages after the evaporation of alkali-halide nanocrystals. The required electron dose was approximately 1 to 20 C/cm2 at 120 keV at room temperature, which was about two orders of magnitude smaller than that required for conventional beam-induced graphitization. The “knock-on” effect of primary electrons strongly induced the decomposition of the alkali-halide crystal inside the amorphous carbon shell. However, the strong ionic cohesion quickly reformed the crystal into thin layers inside the amorphous shell. The bond excitation induced by the electron beam irradiation seemed to enhance strongly the graphitization at the interface between the outer amorphous carbon shell and the inner alkali-halide crystal.

Electronic excitation in bulk and nanocrystalline alkali halides

The lowest energy excitations in bulk alkali halides are investigated by considering five different excited state descriptions. It is concluded that excitation transfers one outermost halide electron in the fully ionic ground state to the lowest energy vacant s orbital of one closest cation neighbour to produce the excited state termed dipolar. The excitation energies of seven salts were computed using shell model description of the lattice polarization produced by the effective dipole moment of the excited state neutral halogen-neutral metal pair. Ab initio uncorrelated short-range inter-ionic interactions computed from anion wavefunctions adapted to the in-crystal environment were augmented by short-range electron correlation contributions derived from uniform electron-gas density functional theory. Dispersive attractions including wavefunction overlap damping were introduced using reliable semi-empirical dispersion coefficients. The good agreement between the predicted excitation energies and experiment provides strong evidence that the excited state is dipolar. In alkali halide nanocrystals in which each ionic plane contains only four ions, the Madelung energies are significantly reduced compared with the bulk. This predicts that the corresponding intra-crystal excitation energies in the nanocrystals, where there are two excited states depending on whether the halide electron is transferred to a cation in the same or in the neighbouring plane, will be reduced by almost 2 eV. For such an encapsulated KI crystal, it has been shown that the greater polarization in the excited state of the bulk crystal causes these reductions to be lowered to a 1.1 eV-1.5 eV range for the case of charge transfer to a neighbouring plane. For intra-plane charge transfer the magnitude of the polarization energy is further reduced thus causing the excitation in these encapsulated materials to be only 0.2 eV less than in the bulk crystal.

In situ Observation of Deliquesced Droplets of Alkali Halide Nanocrystals: Investigation of Ion Segregation in Droplets for Environmental Science

Anion concentrations at the air/water interface of saline droplets are important in atmospheric and environmental chemistry, because gaseous halogens emitted from the droplet surface mediate various key tropospheric chemical processes. In this study, the adsorption of water on alkali halide nanocrystals (KBr, KCl, KF, NaCl) on SiO2 and their deliquescence was in-situ investigated by noncontact atomic force microscopy (AFM) in an amplitude-modulation mode based on electrostatic forces. We also used ambient-pressure X-ray photoelectron spectroscopy in a synchrotron facility to complement our AFM results. For KBr, KCl and NaCl, deliquesced droplets show negative surface potentials relative to the surrounding region, indicating the preferential segregation of Br- and Cl- anions to the air/solution interface, even in the presence of a liquid/solid interface located a few nanometers away. This enhancement of anion concentration at the thin-droplet surface is more drastic for larger anions, meaning that heterogeneous reactions of gas-phase molecules with saline droplets to emit gaseous halogens can be more significant with larger anions.

Ion Segregation and Deliquescence of Alkali Halide Nanocrystals on SiO2

The adsorption of water on alkali halide (KBr, KCl, KF, NaCl) nanocrystals on SiO2 and their deliquescence was investigated as a function of relative humidity (RH) from 8% to near saturation by scanning polarization force microscopy. At low humidity, water adsorption solvates ions at the surface of the crystals and increases their mobility. This results in a large increase in the dielectric constant, which is manifested in an increase in the electrostatic force and in an increase in the apparent height of the nanocrystals. Above 58% RH, the diffusion of ions leads to Ostwald ripening, where larger nanocrystals grow at the expense of the smaller ones. At the deliquescence point, droplets were formed. For KBr, KCl, and NaCl, the droplets exhibit a negative surface potential relative to the surrounding region, which is indicative of the preferential segregation of anions to the air/solution interface.

A theoretical study of the cohesion of noble gases on graphite

The interactions of the noble gases with a graphene sheet are investigated theoretically. The short range repulsive interaction between the noble gas and each carbon atom is described using Hartree-Fock atomic densities and a local density functional theory with the exchange functional corrected for the finite range of the interaction by introducing a Rae-type correction depending on the effective number of electrons. The long range interactions are introduced as the sum of the Axilrod-Teller triple-dipole interaction plus the dipole-dipole and dipole-quadrupole dispersive attractions damped according to the theory of Jacobi and Csanak. The energy arising from the interactions between the permanent quadrupoles on the carbon atoms with the dipole they induce on the noble gas is negligible, being nonzero only on account of the atomistic structure of graphene. The mobile and delocalized nature of the graphene pi electrons causes the effective number of electrons to be around 500 rather than that of 12 appropriate for a system of entirely localized interactions with individual carbon atoms. Inclusion of the Axilrod-Teller term is required to obtain reliable predictions for the binding energies and equilibrium geometries. Absorption of a noble gas atom is predicted to occur at the site above the center of a six membered ring although this is preferred over two other sites by only about 5 meV. The methods presented for generating all the potentials can be applied to derive the interactions between any ion and carbon atom in the wall of a single-walled nanotube. Knowledge of these interactions is required to study the alkali halide nanocrystals encapsulated in single-walled carbon nanotubes of current interest.

Structure and energetics of LiF chains as a model for low dimensional alkali halide nanocrystals

Nanocrystals of inorganic salts constrained in geometry by encapsulation in single walled carbon nanotubes have been shown by recent high-resolution transmission electron microscopy experiments to exhibit specific distortions of their structure in comparison with the ideal bulk. These structural distortions of low dimensional crystallites are analyzed within the Hartree–Fock approximation and density functional theory based on the simplest alkali halide chains, LiF. The observed patterns of distortions in the structure of nanocrystals are the results of low dimensionality and the axial strain (end effects). The structural trends calculated ab initio for LiF are compared with analytical predictions based on the traditional Born theory of bulk crystals and experiments. The energetics of the LiF crystallites is discussed.

Fundamental Global Model for the Structures and Energetics of Nanocrystalline Ionic Solids

This paper presents a general theory elucidating the relationships between the structures and cohesive energetics of alkali halide nanocrystals consisting of small sections of bulk rocksalt structures with m(1) and m(2) rows but infinite along the z axis. The theory introduces the electrostatic interactions between the ions treated as point charges and the short-range repulsions between the closest ion neighbors with the latter terms written in the Born form Ar(-)(n). Minimum energy structures are defined by the distances a(e) and b(e) separating the closest ions perpendicular and parallel to the z direction. The ratio a(e)/b(e), defining the crystal shape, is independent of the strength A of the short-range repulsion, greater than the bulk value of unity, and increases with decrease of the crystal cross section or n. This ratio tends toward unity in the hard sphere limit of infinite n. Both b(e)/R(6:6)(e) and a(e)/R(6:6)(e), with the bulk separation R(6:6)(e), are less than one, increase with increase of the crystal cross section or n, and are independent of A if this is independent of structure. The structural dependence of A increases its value with a decreasing crystal cross section rendering closer to unity the ratios a(e)/b(e), b(e)/R(6:6)(e), and a(e)/R(6:6)(e). Energy gains on relaxing the crystal toward equilibrium from its bulk separations decrease with increase of the crystal cross section or n, being about 60 kJ/mol for a one-dimensional chain with n = 6 but 0.5 kJ/mol for m(1) = m(2) = 4 with n = 12. The energy gained on relaxing to a structure with a(e)/b(e) constrained at unity is about 10 times greater than the further energy gains consequent on removing this constraint. The present theory neglecting the interaction between ions and the encapsulating nanotube explains the experimentally measured b(e)/R(6:6)(e) ratios. The observation that the a(e)/R(6:6)(e) values are greater than one shows that ion-wall interactions are important in determining the values of a(e).

Effect of Lattice Energy Mismatch on the Relative Mass Peak Intensities of Mixed Alkali Halide Nanocrystals

The relative mass peak intensity distribution of the [M14-nAnI13]+ mixed alkali halide nanocrystals containing a “magic” number of 14 metal cations (M and A) and 13 iodide anions is examined. These...

Dynamics of Formation and Evaporation of Mixed Alkali Halide Nanocrystals: A Case of Comparable Lattice Energies

Abstract : The relative intensity distribution of the (Rb(n)K(14-n)I(13)(+) mixed nanocrystals containing a 'magic' number of 14 metal cations and 13 iodide anions is examined. These nanocrystals were generated through sputtering of mixed RbI/KI solids using fast atom bombardment, and analyzed by use of a double-focusing sector-field mass spectrometer. The relative intensities of the observed peaks is found to have a 'bimodal' distribution with a maximum near n=7. This observation is explained by assuming: (a) a rate of formation that is proportional to the number of isomers that can be formed for each mixed nanocrystal, and (b) a rate of dissociation that is insensitive to the composition of the mixed nanocrystal. The later assumption leads to the conclusion that the internal energies of these mixed nanocrystals are similar and independent of their composition. The conclusion is supported by the results of Monte Carlo simulation annealing as well as by the similarity of the known lattice energies of KI and RbI bulk crystals.

Adsorption reactions of alkali-halide nanocrystals: Identification of an important surface defect

The adsorption reactions of alkali-halide clusters have been investigated on a size-selected basis using flow-reactor methods. The reactivity of larger sodium-fluoride clusters [NanFn−1]+ toward polar molecules NH3 and H2O shows a distinctive pattern as a function ofn at ambient temperature. Comparison with computed structures shows that aparticular kind of defect greatly facilitates the initial adsorption process. This defect can be formed by removal of an ion-pair from the face of an otherwise perfect nanocrystal, to create a basket-like opening for the adsorbed molecule. It is shown that this kind of defect occurs as a most stable low-temperature structure only to a certain size, after which a less reactive defect takes its place. The implications of these findings for the adsorption reactivity of the surfaces of ionic solids are briefly discussed.

Photon-stimulated ejection of atoms from alkali-halide nanocrystals.

Interband excitation of single alkali-halide nanocrystals---in the form of mass-selected ${\mathit{M}}_{\mathit{n}\ifmmode\pm\else\textpm\fi{}1}$${\mathit{X}}_{\mathit{n}}^{\ifmmode\pm\else\textpm\fi{}}$ beams (M=K, Cs and X=Br I)---by ultraviolet radiation leads exclusively to halogen-atom emission with very large cross section. The final state is a midgap (F-center) state. A pathway consisting of direct ejection of a photoneutralized surface atom is derived from a band picture using the computed energetics of the (3,3,3) nanocrystal ${\mathit{M}}_{14}$${\mathit{X}}_{13}^{+}$. This mechanism is contrasted with photon-stimulated desorption from crystal surfaces, and related to formation of surface defect states and metallized overlayers.

Impact-Induced Cleaving and Melting of Alkali-Halide Nanocrystals

Impact of nanocrystalline alkali-halide clusters against solid surfaces causes them to fission exclusively into low surface-energy fragments. In time-of-flight scattering experiments, this process appears at an impact energy so low that it must result from a single-step cleavage of the nanocrystal along low surface-energy cleavage planes. At higher energies (more than 1 electron volt per atom), a crossover occurs to an entirely different behavior-evaporative cascades that proceed irrespective of the structureenergetic properties of the fragments. These cascades, and the approximately linear scaling of the crossover energy with cluster size, are characteristic of impact-induced transformation of the cluster to a molten state. Collision with the high-rigidity surface of silicon gives a substantially greater cleavage probability than the soft basal-plane surface of graphite.