Structural and electronic properties of molybdenum chalcohalide nanowires Igor Popov, Teng Yang, Savas Berber, Gotthard Seifert, David Tománek Technical University Dresden Michigan State University SLONANO 2007 Ljubljana,

Some notes on previous work Potel et al. in 1980:  first synthesis of M 2 Mo 6 Se 6 nanowires (M=Na,K,In,Tl)  observed quasi-1D metallic character T.Hughbanks and R. Hoffmann:  thorough theoretical study of Mo n S m clusters and Mo 6 S 6 nanowire as infinte extensions of Mo 6 S 8 cluster  bands characterized using the group theory approach  Mo 4d character of bands around Fermi level Ribeiro et al.:  DFT calculations of the interstitially doped nanowires  main effect of alkali dopants is shifting of Fermi level * M.Potel et al. J.Solid State Chem. 35, 286 (1980) + T. Hughbanks and R. Hoffmann J. Am. Chem. Soc. 1983, 1150 (1983) - F.J. Ribeiro et al. PRB 65, (2002)

Some notes on previous work L. Venkataraman et al.:  Synthesis of isolated Mo 6 Se 6 nanowires – dissolution of the Li 2 Mo 6 Se 6 crystals  STM and STS measurements  metallic, well conductive systems  results suggest no Peierls transition and the wires remain metallic at 5K S. Gemming et al.:  DFT study  interstitially doped Mo 6 S 6 nanowires with Li chains  lattice parameter increases with doping  metallic wires with dominant Mo 4d conduction channels around Ef  bands’ dispersion is negligible in directions orthogonal to the nanowires + L. Venkataraman, C.M.Lieber, PRL 83,5334 (1999) * S. Gemming, G. Seifert, I.Vilfan, PSS 243, 3320 (2006)

Some notes on previous work I. Vilfan:  Mo 6 S 6 nanowires (without doping)  structural, mechanical and electronic properties Y. Teng, I. Vilfan:  Mo 6 S 9-x I x nanowires  interesting mechanical and electronic properties of the accordion-like structures M.I. Ploscaru et al:  synthesis of networks of Mo 6 S 9-x I x nanowires, interconnecting gold clusters Understanding and research on Molybdenum-chalchohalide structures is still in it’s infancy w.r.t. research on Carbon nanotubes (CNTs) * I. Vilfan, EPJB 51, 277 (2006) + T.Yang wt al, PRL 96, (2006) - M.I Ploscaru et al. Nanololett. 7, 1445 (2007)

Idea of our research  Exploit substitutional doping with Iodine for two aims:  substitutional doping has same effects as interstitial ?  stabilization of the nanowire ?  free-standing nanowire (opposite to 3D crystal when interstitial doping is used)  Analyze electronic and transport properties of such wires, and compare them with the corresponding systems.

Theoretical machinery DFT code – SIESTA package  Perdew-Zunger form of Exc in LDA  Troullier-Martins pseudopotentials with core corrections included  double-zeta basis including Mo5p orbitals  8 k-points along nanowires in the reciprocal lattice Preoptimizations of the geometries with one or two unitcells with the faster density-functional based tight binding method (DFTB). Potential energy surface of the bundle: DFTB augmented with Van der Waals interaction.

A subset of investigated geometries More than 30 isomers with various arrangements of Iodine dopants are fully optimized simultaneously with their lattice parameters.

Stability of the nanowires, a “magic” stability of x=2 stoichiometry allows selectivity of a nanowire with x=2 Iodine concentration. Energy gain with respect to the average binding energy. The lattice parameter monotonically increases with doping. A possible synthesis pathway:

Axial stiffness Energy normalized by the number of Mo atoms in MoSI nanowire and by number of C atoms in CNT.  the MoSI wire have similarly high stiffness as CNT.

Optimized geometry of Mo 6 S 6 isomer The same optimized geometries (lat. params. and bondlengths) like in the previous research.

Structural changes of Mo 6 S 4 I 2

Structural properties A new sublattice – consequence of local change of the crystal potential along the Iodine chains. This has important effect on electronic structure of the isomer.

Binding energy in bundles Binding energy of the bundle with respect to isolated nanowire (DFTB + VdW) The most stable configuration occurs at d=9.3A when the binding energy is 0.1 eV for 1Ǻ long nanowire segment. This corresponds to the energy of CNT bundle. Due to high anisotropy of MoSI nanowires, average attraction is smaller, possibly even repulsive  much easier to separate the free-standing nanowires than CNTs.

DFTB + van der Waals interaction. Small dispersion in directions orthogonal to axis of nanowires  The interwire interaction is almost exclusively of van der Waals type with negligible chemical bonding.  The same results obtained by I. Vilfan and S. Gemming for K 2 Mo 6 S 6 and Li 2 Mo 6 S 6 nanowire bundles. Band structure of the bundle

Electronic properties – comparison to CNTs With increase of the I concentration Fermi level shifts up on energy scale. The dispersion of bands does not change significantly with doping. New flat band at Fermi level is the consequence of locally changed potential along Iodine lines, i.e. newly formed sub-lattice in Mo-backbone.  position of the new band depends sensitively on lattice param. At folding point (X) bonding and antibonding states meet  electronic origin for the pronounced stability of Mo 6 S 4 I 2 nanowire. The two bands are linear like in (n,n) CNT C 2p  Mo 4d   indication for massless Dirac fermions like in (n,n) CNTs. Separation of the two closest van Hove singularities matches the case of (13,13) CNT. All states around Ef have Mo-origin  conductance involves mostly the Mo-backbone

Upper band ρ STM = 2*10 -4 el/a 0 3 (E F +1.0, E F +1.5 eV) Wire direction Lower band ρ STM = 2*10 -4 el/a 0 3 (E F +0.3, E F +1.0 eV) E(k) for x = 0, equilibrium structure Mo 6 S 6

I 5p y / I5p z hybridize with Mo: 4d xy + 4d xz Mo: 4d I: 5p th STM in 74th band Wire direction

STM in 75th band Wire direction Cross section p y + p z dz2dz2 75 th d xy +d xz Mo: 4d xy + 4d xz I: 5p y + 5p z - +

Conclusions  Investigated a large number of Mo 6 S 6-x I x isomers  The most stable is x=2 isomer  It’s “magic” stability is due to unique electronics  Nanowires with completely different chemical nature, but with remarkably similarities with Carbon nanotubes  stiffness  electronic and transport properties  Advantages over CNTs:  Easier separation of single- and free-standing nanowires  Always metallic, dependless on isomer and stoichiometry  Termination of finite wire segments with Sulfur atoms  Easier binding as thio groups with gold electrodes  I.Popov, T.Yang, S.Berber, G.Seifert, D.Tomanek, PRL 99, (2007)