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, 10.10.2007.

Some notes on previous work

• Potel et al. in 1980:

first synthesis of M2Mo6Se6 nanowires (M=Na,K,In,Tl)

observed quasi-1D metallic character

• T.Hughbanks and R. Hoffmann:

thorough theoretical study of MonSm clusters and Mo6S6 nanowire as infinte extensions of Mo6S8 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, 153401 (2002)

Some notes on previous work

• L. Venkataraman et al.:

Synthesis of isolated Mo6Se6 nanowires – dissolution of the Li2Mo6Se6 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 Mo6S6 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:

Mo6S6 nanowires (without doping)

structural, mechanical and electronic properties

• Y. Teng, I. Vilfan:

Mo6S9-xIx nanowires

interesting mechanical and electronic properties of the accordion-like structures

• M.I. Ploscaru et al:

synthesis of networks of Mo6S9-xIx 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 96,125502 (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 Mo6S6 isomer

2.7

2.5

2.7

2.5

The same optimized geometries (lat. params. and bondlengths) like in the previous research.

Structural changes of Mo6S4I2

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 K2Mo6S6 and Li2Mo6S6 nanowire bundles.

Band structure of the bundle

Electronic properties – comparison to CNTsWith 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 Mo6S4I2 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/a03

(EF+1.0, EF+1.5 eV)

Wire direction

Lower band

ρ STM = 2*10-4 el/a03

(EF+0.3, EF+1.0 eV)

E(k) for x = 0, equilibrium structure

Mo6S6

I 5py/ I5pz hybridize with

Mo: 4dxy + 4dxz

Mo: 4d

+

+ -

-

I: 5p

+

-

74th

STM in 74th band

Wire direction

STM in 75th band

Wire directionCross section

py+ pz

dz2

75th

dxy +dxz

+

+ -

- +

-

-

- +

+

Mo: 4dxy + 4dxz

I: 5py + 5pz

-

+

Conclusions

Investigated a large number of Mo6S6-xIx 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, 085503 (2007)