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.
Jan 11, 2016
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)