Pathways of Growth of CdSe Nanocrystals from Nucleant (CdSe)₃₄ Clusters

The initial steps in the growth of quantum platelets from the wurtzite-type (CdSe)₃₄ clusters are simulated using density functional theory with the generalized gradient approximation. The nucleant (CdSe)₃₄ cluster has been chosen for simulations because it has experimentally been found to be a magic-size nucleant for the low-temperature growth of CdSe quantum platelets. According to the results of our calculations, the growth is anisotropic and favors the (0001) direction, which is consistent with the experimental findings. We found that growth in other directions lowers the symmetry of the resulting clusters and that the asymmetrical positioning of rhombic defects causes the growing platelet to bend due to the surface strain, which appears to be the limiting factor of growth. An alternative pathway to quantum platelet growth could proceed via the decomposition of (CdSe)₃₄ to (CdSe)₁₃ in electron-donating media, which was found to be thermodynamically favorable. Side product (CdSe)₂₁ generated in this process is capable of growing via hexagonal stacking as well as propagating as a nanotube

Joint Studies of Spin Frustration Induced by Doping Small ZnSe Nanoparticles with Fe Atoms

Abstract As a 1.8 nm ZnSe nanocrystal is progressively doped with 1%, 5%, and 10% Fe, it shows a progressive change in its magnetic properties from a superparamagnetic FM‐dominated exchange type to an onset of AFM exchange with evidence of spin frustration. Magnetization measurements allow to obtain exchange coupling constants that are compared to the results of a Broken‐Symmetry Density Functional Theory (BS‐DFT) model of a doped (ZnSe)34 cluster. DFT shows a capability to reproduce the experimental pattern of the increasing influence of AFM exchange as doping concentration increases. The material phase segregates at the edges where strained rhombic surface sites are the preferred doping sites of iron. Large concentrations of iron leads to the formation of Fe clusters and complex exchange patterns that result in spin frustration in some iron trimers but none in the others. The spin frustration of these complex systems by assuming mirror symmetry of the sites when fitting by using BS‐DFT formalism is classified and analyzed. While some individual J constants obtained have significant errors, the averaged exchange constants are generally in good agreement with our experimental data

Structure and magnetic properties of Fe

The electronic, geometrical, and magnetic structures of iron clusters Fen substituted with a single Gd atom are studied using density functional theory with generalized gradient approximation for n = 12 − 19. An all electron basis set of a triple-ζ quality is chosen for the iron atoms whereas an effective core potential and the basis set of a triple-ζ quality are used for the Gd atom in optimizations of FenGd clusters. The lowest total energy state of a FenGd cluster was found to possess a geometrical structure where the Gd atom substitutes for a surface Fe atom of the Fen+1 cluster at given n. The total spin of a substituted cluster is larger than the total spin of the lowest total energy state of a unary iron cluster with the same number of atoms. The binding energy per atom in a substituted Fen−1Gd cluster is somewhat smaller than the binding energy per atom in a non-substituted Fen cluster. That is, the Gd substitution increases the total spin magnetic moment but destabilizes substituted iron clusters

Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH3–NO2 structure with low energy barriers. While direct cleavage of the C–N bond from the parent nitromethane cation produces NO2+ and CH3+, rearrangement prior to dissociation accounts for fragmentation products including NO+, CH2OH+, and CH2NO+. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH2+, H+, and NO22+.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane