Frank Hagelberg

A density functional theory investigation of CrSin (n=1–6) clusters

Abstract Clusters of the form CrSi n ( n =1–6) were investigated computationally using a density functional approach. In particular, geometry optimizations were carried out under the constraint of well-defined point group symmetries at the B3LYP level employing a pseudopotential method in conjunction with double zeta basis sets. In this article, the resulting total energies, Mulliken atomic net populations, overlap populations, fragmentation energies and geometries of CrSi n ( n =1–6) are presented and discussed, together with natural populations and natural electron configurations. In addition, we comment on the charge transfer within the clusters. From this analysis, the 3d orbital of the Cr atom in CrSi n ( n =1–6) cluster absorbs electrons. From this tendency, conclusions are drawn with respect to the electronic populations and the chemical bond between Si and Cr as well as Si and Si.

A density functional investigation of MoSin (n=1–6) clusters

Abstract A series of computational investigations on MoSin (n=1–6) clusters is reported. For each unit, geometry optimizations are performed at the B3LYP/LanL2DZ level, subsequently, electronic and bonding features of the resulting equilibrium structure are discussed. The emphasis of this study is placed on the magnetic properties of the MoSiN (N=1–6) systems and, more specifically, the impact of their spin on the internal electron transfer between the Sin subsystem and the Mo atom. As a general trend for MoSiN clusters with N>1, we record a change of the natural charge on the Mo atom from negative to positive as the spin constraint on the system increases from S=0 to S=3. This feature is shown to be associated with a pronounced drop in the stability of the cluster as one goes from the singlet to the septet spin configuration.

Geometric and Electronic Structure of WSiN(N = 1–6, 12) Clusters

Geometry optimizations of WSiN (n = 1–6, 12) clusters are performed using the B3LYP/ LanL2DZ method for a sequence of different spin states, changing from spin singlet to spin septet conditions. The resulting equilibrium structures are discussed under the aspects of geometric features, cluster internal charge transfer and magnetic properties. It is shown that the W impurity in the SiN environment generally acts as an electron acceptor. However, the charge on the W atom, as obtained by natural population analysis, can be sensitively tuned through the variation of the spin constraint from S = 0 to S = 3. The resulting geometries of WSin (N = 3–6) are compared with the known ground state structures of SiN+1 (N = 3–6), and substitutional geometries are identified for N = 3 and N = 5. The nonzero spin states of WSiN are shown to display different patterns of magnetic order, corresponding to uniform and to alternating atomic spin orientations within the cluster. Highly compact Oh and D6h structures are identified as stable geometries of WSi6 and of the experimentally detected unit WSi12, respectively. Comparison is made with the cluster series MoSiN(N = 1–6) and CuSiN(N = 1–6,12).

Do methanethiol adsorbates on the Au(111) surface dissociate

The interaction of methanethiol molecules CH3SH with the Au(111) surface is investigated, and it is found for the first time that the S-H bond remains intact when the methanethiol molecules are adsorbed on the regular Au(111) surface. However, it breaks if defects are present in the Au(111) surface. At low coverage, the fcc region is favored for S atom adsorption, but at saturated coverage the adsorption energies at various sites are almost isoenergetic. The presented calculations show that a methanethiol layer on the regular Au(111) surface does not dimerize.

Free-electron attachment to coronene and corannulene in the gas phase

Electron attachment to the polyaromatic hydrocarbons coronene and corannulene is studied in the electron energy range of about 0-14 eV using a high-resolution crossed electron-neutral beam setup. The major anions observed are the parent anions peaking at about 0 eV with cross sections of 3.8 x 10(-20) and 1 x 10(-19) m(2), respectively. The only fragment anions formed in coronene and corannulene are the dehydrogenated coronene and corannulene anions. Other anions observed in the negative mass spectra at about 0 eV can be ascribed to impurities of the sample. High-level quantum-mechanical studies are carried out for the determination of electron affinities, hydrogen binding energies, and structures of both molecules. The behavior of coronene and corannulene upon electron attachment is compared with that of other polyaromatic hydrocarbons studied previously.

Charge transfer mechanism in Cu-doped silicon clusters: a density functional study

Abstract The geometric and bonding properties of Si n Cu ( n =4, 6) clusters were studied using the ab initio and density functional methods. Three isomers for Si 4 Cu and two isomers for Si 6 Cu were found. The structure of the most stable isomer for Si n Cu keeps the framework of corresponding Si n cluster nearly unchanged. Charge transfer in Si n Cu was examined to proceed from Cu to Si atoms. Population analysis indicates that the 3d shell of Cu atom is only slightly disturbed in Si n Cu and that Cu behaves like an alkali metal atom. The bonding nature in Si n Cu was analyzed by correlating the HOMO of Si n Cu to the LUMO of corresponding Si n cluster, and a dominant s–p or p–p bonding as well as a weak d–p bonding was found for the Cu–Si interaction in Si n Cu. This bonding nature is quite different from the interaction of 3d metals on silicon surfaces or 3d impurities in bulk silicon, where the bonding involving Si 3p and M 3d overlap is dominant. Remarkable similarity is found both in the geometry and in the bonding nature between the corresponding isomers of Si n Cu and Si n Na, confirming the alkali-metal-like behavior of Cu in Si n Cu.

Geometric structures and stabilities of CuSin clusters (n=8,10,12)

Abstract The geometric structures and stabilities of CuSi n ( n =8,10,12) clusters were studied using a hybrid density functional method (B3LYP). Eight isomers were found for CuSi 8 , 21 isomers for CuSi 10 and five isomers for CuSi 12 . The stabilities of CuSi n can be well related to the stabilities of Si n . The Si frameworks are kept nearly unchanged and Cu is located at a substitutional site in many isomers of CuSi n , especially for CuSi 10 . The obtained results can be used to understand well the observations in mass spectrometric experiments. The high abundance of CuSi 10 in the experimental mass spectrum is attributed to the enhanced stability of CuSi 10 over neighboring clusters and the existence of a large number of low-lying isomers.

Quantum Monte Carlo characterization of small Cu-doped silicon clusters: CuSi4 and CuSi6

The relative energies, binding energies, and adsorption energies of three CuSi4 and two CuSi6 clusters have been computed in the fixed-node diffusion Monte Carlo (FNDMC), CASSCF, and B3LYP DFT methods. These results are compared with the earlier Hartree–Fock (HF) and B3LYP DFT investigations of these systems by two of us [C. Xiao and F. Hagelberg, J. Mol. Struct.: THEOCHEM 529, 241 (2000)]. The very close energy level spacing of the isomers under consideration confirms the previous work of Xiao and Hagelberg. The FNDMC results show some qualitative discrepancies with B3LYP DFT, and HF findings. They also confirm the appropriateness of the B3LYP DFT method for the prediction of the most stable CuSi4 isomer, while the CASSCF method compares more favorably with FNDMC for adsorption energies than B3LYP DFT.

THEORETICAL INVESTIGATIONS ON CLOSED-SHELL SILICON CLUSTERS DOPED WITH CU ATOMS

Abstract Silicon clusters doped with a single copper impurity (CuSiN), which were detected previously by mass spectrometric experiment, are explored by means of ab initio analysis. Features related to geometries, stabilities and adsorption energies of the species CuSiN, with N=4, 6, 8, 10, 12, 14 are discussed. The sensitive dependence of the physical properties of CuSiN clusters on the geometric arrangement of the respective SiN subsystem, as emerging from our research, is emphasized.

Impact of Tube Curvature on the Ground-State Magnetism of Axially Confined Single-Walled Carbon Nanotubes of the Zigzag-Type

The magnetic properties of axially confined, hydrogenated single-walled carbon nanotubes (SWCNTs) of the (n,0)-type with n=5-24 are systematically explored by density functional theory. Emphasis is placed on the relation between the ground-state magnetic moments of SWCNTs and zigzag graphene nanoribbons (ZGNRs). Comparison between the SWCNTs considered here and ZGNRs of equal length gives rise to two basic questions: 1) how does the nanotube curvature affect the antiferromagnetic order known to prevail for ZGNRs, and 2) to what extent do the magnetic moments localized at the SWCNT edges deviate from the zero-curvature limit of n/3μB ? In response to these questions, it is found that systems with n≥7 display preference for antiferromagnetic order at any length investigated, whereas for n=5, 6 the magnetic phase varies with tube length. Furthermore, elementary patterns are identified that describe the progression of the magnitude of the magnetic moment with n for the longest tubes explored in this work. The spin densities of the considered SWCNTs are analyzed as a function of the tube length L, with L ranging from 3 to 11 transpolyene rings for n≥7 and from 3 to 30 rings for n=5 and 6.