The formation and ejection of endohedral Cs@C60+ by low energy collisions (35–220 eV) of Cs+ ions with surface adsorbed C60 molecules

Anion-encapsulating fullerenes behave as large anions: a DFT study

M06L/6-311++G(d,p)//M06L/6-31G(d,p) level density functional theory studies show that the endohedral reaction of C60 with X- (X = F, Cl, Br, OH, NH2, NO2, CN, and ClO) is exothermic by 37.8-65.2 kcal mol-1. The exothermic character of the reaction is drastically reduced in polar and nonpolar solvents due to the lack of direct solvation influence on the encapsulated anion. In all X-@C60, the occupied frontier molecular orbitals (FMOs) are located on X- while the energy levels of FMOs centered on C60 are very similar to those of the C60- radical anion. Molecular electrostatic potential (MESP) analysis of X-@C60 revealed that the negative character of the MESP minimum (Vmin) on the carbon cage increases by ∼72 fold compared to C60, which is very similar to the enhancement in the negative MESP observed on the C60- radical anion. The MESP data and quantum theory of atoms in molecules (QTAIM) analysis of charge, electron delocalization index, and Laplacian of bond critical point (bcp) support significant electron sharing from the anion to the carbon atoms of the fullerene cage, which makes the cage behave like a very large anion in a closed shell configuration. The data are also supportive of a multicenter charge-shift type of bonding interaction between the anion and the carbon cage. The anionic nature of the fullerene cage has been verified in the cases of larger systems such as Cl-@C70, Cl-@C84, and Cl-@C90. The binding of a counter cation K+ with X-@C60 is found to be highly exothermic (∼72 kcal mol-1) and very similar to the binding of K+ with the C60- radical anion (72.9 kcal mol-1), which suggests that C60 in X-@C60 behaves as a closed shell anion.

Generation of M@C(2n)+ (M = K, Rb, Cs, 2n = 80-220) by laser ablation of graphene

RATIONALE

The yield of endohedral metallofullerene (EMF) ions is greatly related to the encaged metal atoms. For alkali metals, the generation of corresponding large-sized monometallofullerene ions by traditional methods is still difficult. The aim of this work is to study the generation and the distribution of alkali-metallofullerene ions by the method of laser ablation with graphene as the precursor.

METHODS

The experiment was performed with a Fourier transform ion cyclotron (FTICR) mass spectrometer. Mass spectra were obtained by laser ablation of the mixtures of graphene and KCl, RbCl or CsCl in the positive ion mode. Collision-induced dissociation (CID) experiments were also performed for some selected ions.

RESULTS

EMF ions of M@C(2n)+ (M = K, Rb, Cs, 2n = 80-220) can be generated by using graphene as the precursor. Other EMF ions with smaller sizes, including M@C60+, were also observed under the optimized experimental conditions. CID experiments of some selected ions confirmed their endohedral structures.

CONCLUSIONS

Metallofullerene ions of M@C(2n)+ (M = K, Rb, Cs, 2n = 80-220) were generated by laser ablation of graphene and corresponding metal salts. Compared with previous results of alkaline earth metals, the yields of EMF ions are generally lower. However, for large-sized EMF ions, the relative intensities of EMF ions to their corresponding fullerene ions I(M@C(2n)+)/ I(C(2n)+) generally increase with the increase in cluster sizes, which is opposite to the trend observed for alkaline earth metals.

Determination of recombination coefficients for nanocrystalline silicon embedded in hydrogenated amorphous silicon

The spectroscopic pump-probe reflectance method was used to investigate recombination dynamics in samples of nanocrystalline silicon embedded in a matrix of hydrogenated amorphous silicon. We found that the dynamics can be described by a rate equation including linear and quadratic terms corresponding to recombination processes associated with impurities and impurity-assisted Auger ionization, respectively. We determined the values of the recombination coefficients using the initial concentrations method. We report the coefficients of 1.5×10(11) s(-1) and 1.1×10(-10) cm(3) s(-1) for the impurity-assisted recombination and Auger ionization, respectively.

The dynamics of endohedral complex formation in surface pick-up scattering as probed by kinetic energy distributions: Experiment and model calculation for Cs@C60+

Endohedral Cs@C60 molecules were formed by implanting low energy (E0 = 30-220 eV) Cs+ ions into C60 molecules adsorbed on gold. Both growth and etching experiments of the surface deposited C(60) layer provide clear evidence for a submonolayer coverage. The Cs+ penetration and Cs@C60 ejection stages are shown to be a combined, single collision event. Thermal desorption measurements did not reveal any Cs@C60 left on the surface following the Cs+ impact. The Cs@C60 formation/ejection event therefore constitutes a unique example of a pick-up scattering by endocomplex formation. Kinetic energy distributions (KEDs) of the outgoing Cs@C60+ were measured for two different Cs+ impact energies under field-free conditions. The most striking observation is the near independence of the KEDs on the Cs+ impact energy. Both KEDs peak around 1.2 eV with similar line shapes. A simple model for the formation/ejection/fragmentation dynamics of the endohedral complex is proposed. The model leads to a strong correlation between the vibrational and kinetic energy of the outgoing Cs@C60. The KEDs are calculated taking into account the competition between the various decay processes: fragmentation and delayed ionization of the neutral Cs@C60 emitted from the surface, fragmentation of the Cs@C60+ ion, and radiative cooling. It is concluded that the measured KEDs are heavily biased by the experimental breakdown function. Good agreement between experimental and calculated KEDs is obtained.