Concentration-Dependent Half Metallicity in Mo6SxI10−x NanowiresFerhat Demiray and Savas Berber*

Physics Department, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey

ABSTRACT: We have investigated the structural andelectronic properties of Mo6SxI10−x nanowires for 0 < x < 2using density functional theory calculations. The optimumatomic structures and lattice constants are determined bysequential S substitution of the fully I decorated Mo6I10nanowire. We find a good agreement with the experimentallattice constants. The nanowires with increasing sulfur contentchange from metallic to half-metallic, and finally to semi-conductors for x = 2. The Mo6SxI10−x nanowires may find usein spintronics applications with their tunable magnetic and electronic properties.

■ INTRODUCTION

Molybdenum chalcohalide nanowires1−4 are expected to findapplications in nanotechnology because of their uniquestructural and electronic properties. These nanowires formstable two- and one-dimensional structures with unusualelectronic properties,5,6 including superconductivity andmagnetism.7 These layered or filamentous substances areknown as catalysts.8 Their large Young’s moduli9,10 and smallshear moduli10 should allow their use in tribologicalapplications.11

The nanowires with the formula of Mo6S9−xIx have beenextensively studied.3,5−7,10−15 The stoichiometry of a molybde-num chalcohalide nanowire is tuned by simply adjusting theratio of elements in the synthesis. Among the molybdenumchalcohalide nanowires, the Mo6S2I8 was first reported byPerrin and Sergent.1 Recently, Raman spectra of this wire werereported,16 and the sulphurization was utilized to obtain newinorganic nanostructures.17 However, a systematic theoreticalwork has not been reported yet. To determine the detailedatomic structure of Mo6S2I8 nanowires, we start from fully Idecorated Mo6I10 nanowire and follow the lowest energystructures while sequentially sulphurizing the nanowire. Thus,we determined the atomic, electronic, and magnetic structuresof Mo6SxI10−x nanowires for 0 < x < 2. Surprisingly, we find thatMo6SxI10−x nanowires have very rich electronic propertiestunable by the sulfur content. The interplay between the sulfurcontent and their magnetic properties may lead to applicationsof these nanowires in spintronics.The common feature in the molybdenum chalcohalide

nanowires is the occurrence of Mo6 octahedra decorated andbridged by chalcogen and halide atoms. The primitive unit cellcontains two octahedra rotated by 180° with respect to eachother along the wire axis. The detailed atomic structure is oftendebated7,12,13 because of the large number of possible sites inwhich the chalcogen and halide atoms can be placed. Thenanowire with the formula of Mo6S2I8 is a rare polymorph ofthe molybdenum chalcohalide nanowires and can besulphurized17 fully to give MoS2 nanotubes. Inspired by the

sulphurization experiments, we determined the atomic structureof Mo6S2I8 nanowire through successive sulphurization of thefully iodine decorated Mo6I10 nanowire that has the mainstructural features of the molybdenum chalcohalide nanowires.

■ COMPUTATIONAL METHOD

In this work, we used the local density approximation (LDA) todensity functional theory (DFT) as implemented in theSIESTA code.18 We utilized the Perdew−Zunger19 form ofthe exchange-correlation functional, norm-conserving Troul-lier−Martins pseudopotentials20 with partial core corrections inthe Kleinman−Bylander fully separable form,21 and a double-ζbasis set augmented by polarization orbitals. The nanowires areplaced with at least 16.4 Å interwire separation in a tetragonalunit cell that has the lattice constant a along the nanowire axis.The Brillouin zone was sampled with a 1 × 1 × 6 Monkhorst−Pack k-point grid. The charge density and potentials wereexpressed on a real-space grid with a mesh cutoff energy of 200Ry. The structure relaxations in conjugate-gradient algorithmwere continued until all force components are less than 0.04eV/Å. In combination with the structural relaxations, the latticeconstant a was scanned with the interval of 0.1 Å.

■ RESULTS AND DISCUSSION

Atomic Structure and Energetics. The atomic structureof a fully iodine decorated nanowire Mo6I10 is depicted in theleft panel of Figure 1a. The binding energies of optimizedstructures at different lattice constant values a are shown in theright panel of Figure 1a and indicate that the Mo6I10 nanowireis a soft ductile material with a shallow energy minimum. In theoptimum structure, the equilibrium lattice constant aeq is 13.0Å, the Mo−Mo bond length ≈ 2.7 Å, and the Mo−I bondlength ≈ 2.9 Å for the bridging I atoms and ∼2.78 Å for theother iodine atoms.

Received: September 12, 2012Published: October 22, 2012

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© 2012 American Chemical Society 23833 dx.doi.org/10.1021/jp309070s | J. Phys. Chem. C 2012, 116, 23833−23837

Starting from the Mo6I10 nanowire structure, shown in Figure1a, we substituted the iodine with sulfur for all uniqueconfigurations and obtained optimized structures whileconstraining the value of the axial lattice constant a. We keptthe lowest energy structure moving from an S concentration xto its next value. The possible substitution positions are ligand(A), bridging (B), and central (C) positions, and are shown inthe left panel of Figure 1a. In the following, we discuss the

properties of Mo6SxI10−x nanowires, where sulfur was used tosubstitute for iodine.First, we substituted an iodine atom with a sulfur atom and

considered three unique positions for the substituent, which areligand (A), bridging (B), and central (C) positions as defined inFigure 1a. The sulfur atom prefers the central position (C) thathas 2.55 eV lower energy than that of the ligand position (A),which is the second best substitution site. Having determined

Figure 1. Optimized atomic structures of Mo6SxI10−x nanowires are depicted for (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5, and (e) x = 2 in the leftpanels. The blue (dark) color is used for Mo atom, the purple (dark gray) color for I atom, and the yellow (light gray) for the S atom. In the rightpanels, the binding energy per unit cell is plotted as a function of the lattice constant a.

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the optimum substitution site, we optimized the structures for x= 0.5 at constrained axial lattice constant a values andcalculated the binding energy Eb as a function of a. Becausethe introduction of an S atom to the central (C) positionresults in two different bridges between the octahedra, the Ebdata shown in Figure 1b exhibit two local minima. We find theequilibrium lattice constant aeq to be 12.5 Å, which is lowerthan that of Mo6I10 nanowire. The relaxed atomic structure at

aeq, which is depicted in Figure 1b, reveals that the bridgearound the central S atom is shorter.To find the structure of Mo6S1I9, we started from the

optimum structure for x = 0.5 and exhausted all possiblesubstitution positions for the second S atom. We find that thebest position for the second sulfur atom is the other centralsubstitution site, which is energetically more favorable than thesecond best position by 2.41 eV. The Eb as a function of a,shown in Figure 1c, indicates a lower aeq value of 11.9 Å. Theoptimum atomic structure of Mo6S1I9 nanowire, depicted inFigure 1c, includes two equal bridges between the octahedra.Therefore, there should be a single local minimum in the Ebversus a plot. However, the bonds between the central S atomand the neighboring octahedra may be broken in an extremelystretched nanowire. Therefore, the curve in Figure 1c has adiscontinuity.Comparison of the local curvatures of Eb versus a plots in

Figure 1a and c indicates that the inclusion of central S atomhardens the bridge between the octahedra. The Mo−Modistance is decreased to ∼2.68 Å, and the Mo−I distance islowered to ∼2.82 Å for bridging I atoms, while it stays almostthe same value of ∼2.78 Å for the other I atoms. Because thesulfur has the higher oxidation state, the number of electronson the octahedra is reduced and the metal−metal bonding isimproved. The central sulfur atom forces the shorter bridgesthat result in smaller Mo−I distances in the bridge.Continuing successive sulfur substitution, we find that the

next substituent S atom prefers the ligand position (A) to thebridging position (B). The total energy of the ligand positionhas 1.04 eV lower energy than the bridging position for x = 1.5.The optimized structure for x = 1.5 is depicted in Figure 1d.The Eb versus a data in Figure 1d reveal that there is a singleminimum at the lattice constant of aeq = 11.9 Å.The last substituent S atom that leads to Mo6S2I8 nanowire

favors the ligand position (A) that is on the other octahedronand the farthest from the previously substituted ligand position.This position is energetically better by at least 1.15 eV than thebridging position (B). The total energies for different ligandpositions are higher by as much as 0.26 eV when both S atomsare on the same octahedron. However, the energy differencefrom that of the optimum structure is as low as 0.06 eV as longas the two substituent S atoms are on different octahedra.Therefore, we conclude that the S atoms are not in the bridgingposition and substitute iodine atoms at the ligand positionsuniformly along the nanowire in the optimum configuration.However, the translational symmetry in Mo6S2I8 may not bevery strong in experiments. As deduced from the optimizedstructure of Mo6S2I8 nanowire in Figure 1e, the S atoms in

Figure 2. Electronic band structures and density of states (DOS) ofMo6SxI10−x nanowires for (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5,and (e) x = 2. The energies are with respect to the Fermi levels EFdenoted by dashed lines. The negative values are used in plotting theDOS of minority spins. The left panels are band structures of themajority spin, and the middle panels are the minority spin.

Figure 3. The net spin polarization ρmajority(r) − ρminority(r) depicted as3D isosurfaces for (a) x = 0.5, (b) x = 1, and (c) x = 1.5.

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ligand positions form a zigzag chain in the optimumconfiguration.The binding energy Eb as a function of the lattice constant a

for x = 2 is shown in Figure 1e. We find the equilibrium latticeconstant aeq to be 11.7 Å, which is in a reasonable agreementwith the experimental1,22 value of 11.952 Å. Once both centralpositions (C) are occupied by two S atoms, the aeq changesvery little with the increased S concentration x. The Mo atomsbonded to the S atoms in the ligand positions (A) form shorterMo−Mo bonds of 2.65 Å, translating into slightly smaller aeqvalues. The central S atoms are crucial in reaching an agreementbetween calculated and measured lattice constant valuesbecause aeq is determined mainly by the bridges. Anaccordion-like behavior7 of the nanowire exists for x = 0.5,but disappears once the central S atom strengthens the bridgefor x = 1, 1.5, and 2.Comparing the ranges of Eb values in Figure 1, we find that

the binding energy of our 32 atom unit cell increases withincreasing sulfur concentration x because Mo−S bonds arestronger than Mo−I bonds. This bond strength differenceallows the full sulphurization of these nanowires to form MoS2nanostructures.17 Our calculated binding energy per atom,omitting the bonding strength differences of elements, is in therange of 4.4−5.2 eV, explaining the high stability of thesenanowires.1,22

Electronic and Magnetic Structure. The electronicstructures of Mo6SxI10−x nanowires with different compositionsare presented in Figure 2. Our spin polarized electronicstructure calculations indicate that the magnetic coupling inthese nanowires becomes ferromagnetic in the intermediaterange of 0 < x < 2, although the electronic structure is not spinpolarized for fully I decorated Mo6I10 nanowire and for Mo6S2I8nanowire, which has the highest S composition. In Figure 2, theleft panels are the band structures for majority spin and themiddle panels are for minority spin, and the energies are withrespect to the Fermi level EF. The net spin polarizations ρnet(r)= ρmajority(r) − ρminority(r) in these systems are depicted as 3Diso-surface plots in Figure 3. The atomic structure is overlaid inFigure 3, and the atom types are denoted by the colors used inFigure 1.The electronic structure of Mo6I10 nanowire in Figure 2a

shows metallic character, and the Fermi level EF crosses band eat the X point of the Brillouin zone. The electronic structurehas bands with peculiar dispersion near the Fermi level EF, andthere is a free-electron-like band about 2 eV. The nature ofband e is depicted in Figure 4a as 3D contour plots of local

density of states, calculated as the summation of wave functionover double degenerate band e, for both majority and minorityspins. Because band e is related to the octahedra and there aretwo identical octahedra rotated with respect to each other, bande is double degenerate and folded at zone boundary X. Thedispersion of band e is due to the interaction between theoctahedra through the I atoms in the bridge and in the centralsite.When an S atom substitutes the I atom at the central

position, the perfect matching of band e at X point is removedbecause of the broken translational symmetry. Band e shiftsdown, and the band above shifts up because of the strongerbonding of the S atom; both bands remain double degeneratebecause they are still electronic states on two identicaloctahedra. Next, the sulphurization reduces the number ofelectrons on the octahedra by 1, and thus band e should be half-occupied for x = 0.5. Instead, exchange splitting induces a totalspin polarization of 1 μB per unit cell, and only the band e of theminority spin is half-occupied while the band is fully occupiedfor the majority spin. The spatial distribution of the spinpolarization for x = 0.5 is depicted in Figure 3a as a 3Disosurface. The local densities of states for the occupied statesin band e are shown in Figure 4b as 3D contours and indicatethat the states near the X point of the Brillouin zone arelocalized only on one of the octahedra in the real space. Wefind that the net spin polarization in Figure 3a is identical to thedifference between local densities of states for majority andminority spins, depicted in Figure 4b. Therefore, the magneticstructure can be simply understood by the changes in band eand its occupation level. Net majority spin density occurs onthe Mo atoms bonding to the central I atom. The Mo6S0.5I9.5nanowire has a band gap Eg = 0.18 eV for majority spin, and itis metallic for the minority spin.The substitution of an S atom at the second central position

in the nanowire of x = 1 withdraws one more electron from theoctahedra, and thus band e becomes unoccupied for theminority spin. Because the translational symmetry is recoveredfor x = 1, the perfect matching of valence and conduction bandsat X point reoccurs, while the dispersion of these bands differsfrom that of band e. The net spin polarization of 2 μB isdistributed over the octahedra, and the magnetic coupling isferromagnetic as perceived in Figure 3b. The system is half-metallic for x = 1, and the band gap of Eg = 1.68 eV forminority spin is observed.Further substitution of S atoms at ligand positions (A)

eliminates the degeneracy of the electron states on the

Figure 4. The local density of states, which is calculated as the summation of wave function squares in the range of EF −0.4 eV < E < EF, is depictedas 3D isosurfaces for (a) x = 0, (b) x = 0.5. The left panels are for the majority spin (↑), and right panels are for the minority spin (↓).

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octahedra, and more electrons are drawn from the states on theoctahedra. Therefore, the net spin polarization is first reducedto 0.65 μB for x = 1.5, and it disappears for x = 2. For x = 1.5,the band gap does not exist for majority spin, while Eg = 1.46eV for minority spin electrons. Deduced from Figure 3c, themagnetic coupling is ferromagnetic for x = 1.5. The depletionof the states of octahedra opens up a gap of Eg = 1.49 eV in theelectronic structure of Mo6S2I8 nanowire.We find the nanowires to be half metallic except for x = 0

and x = 2, and thus Mo6SxI10−x may find use in spintronics. Ourcalculations univocally showed that adjusting the S concen-tration can be used to tune the magnetic and electronicproperties. Therefore, Mo6SxI10−x nanowires with adjustedcomposition may be exploited in future experiments tosynthesize novel materials for spintronics.

■ SUMMARY AND CONCLUSIONS

We have investigated the structural and electronic properties ofMo6SxI10−x nanowires for 0 < x < 2 using ab initio calculations.The optimum atomic structures and lattice constants aredetermined by sequential S substitution of the fully I decoratedMo6I10 nanowire. We find a good agreement with theexperimental lattice constants. The nanowires with increasingsulfur content change from metallic to half-metallic, and finallyto semiconductors for x = 2. The Mo6SxI10−‑x nanowires mayfind use in spintronics applications with their tunable magneticand electronic properties.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: savasberber@gyte.edu.tr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We acknowledge financial support from The Scientific andTechnological Research Council of Turkey (TUBITAK) undergrant no. 108T740.

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