Phase-Controlled Growth of One-Dimensional Mo6Te6 Nanowires

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Phase-Controlled Growth of 1D MoTe Nanowires and 2D MoTe Ultrathin Films Heterostructures 2

Yayun Yu, Guang Wang, Yuan Tan, Nannan Wu, Xue-Ao Zhang, and Shiqiao Qin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03058 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Phase-Controlled Growth of 1D Mo6Te6 Nanowires and 2D MoTe2 Ultrathin Films Heterostructures

Yayun Yu1, 2, †, Guang Wang

1, 2, 3, † , *

, Yuan Tan1, Nannan Wu1, Xue-Ao Zhang

1, 3, *

,

Shiqiao Qin1, 3

1

College of Science, National University of Defense Technology, Changsha 410073,

China 2

State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University,

Beijing 100084, China 3

State Key Laboratory of High Performance Computing, National University of

Defense Technology, Changsha 410073, China

†

Yayun Yu and Guang Wang contributed equally to this work.

*

E-mail: [email protected]; [email protected]

Abstracts: Controllable synthesizing 1D-2D heterostructures and tuning their atomic and electronic structures is nowadays of particular interest, due to the extraordinary properties and potential applications. Here, we demonstrate the temperature-induced phase-controlled growth of 1D Mo6Te6 - 2D MoTe2 heterostructures via molecular beam epitaxy. In-situ scanning tunneling microscopy study shows 2D ultrathin films are synthesized at low temperature range, while 1D nanowires gradually arise and dominate as temperature increasing. X-ray photoelectron spectroscopy confirms the good stoichiometry and scanning tunneling spectroscopy reveals the semi-metallic property of grown Mo6Te6 nanowires. Through in-situ annealing, a phase transition


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from 2D MoTe2 to 1D Mo6Te6 is induced, thus forming a semimetal-semiconductor junction in atomic level. An upward band bending of 2H-MoTe2 is caused by lateral hole injection from Mo6Te6. The work suggests a new route to synthesize 1D semi-metallic transition metal chalcogenide nanowires, which could serve as ultra-small conducting building blocks and enable band engineering in future 1D-2D heterostructure devices.

Keywords: Semi-metallic 1D Mo6Te6 Nanowire, Molecular Beam Epitaxy, Scanning Tunneling Microscopy/Spectroscopy, X-ray Photoelectron Spectroscopy, 1D-2D Heterostructures

Integrating 1D nanotubes or nanowires (NWs) into 2D ultrathin films is nowadays of tremendous research interest, as the 1D-2D hybrid nanostructures could exhibit exotic and unexpected properties from either component. As an example, carbon nanotube (CNT) has been combined with graphene to serve as transparent conductors, gate electrodes in vacuum electronics and electrode materials in energy storage devices.1-4 Another important candidate is 1D transition metal chalcogenide (TMC) nanowire, with a general formula of M6X6 (M = Mo or W, while X = S, Se or Te). Different from 2D semiconducting transition metal dichalcogenide (TMDC) monolayers with direct bandgaps around 1~2 eV, 1D M6X6 NWs have been predicted to be intrinsically metallic.5-9 Moreover, such 1D NWs show ideal Ohmic contacts with Au electrodes,10 which could hence act as promising ultra-small conducting


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building blocks or buffer contact layers for decreasing contact resistances between 2D TMDCs and the electrodes in next-generation logical devices. 11-12 To date, Mo6S6 and Mo6Se6 NWs have been successfully synthesized by dissolution of crystalline Li2Mo6Se6,13-14 physical vapor deposition (PVD),15 sulfurization of Mo films with benzenethiol16 and electron-beam irradiation of TMDC monolayers.17-19 Recently, Mo6Te6 NWs have also been formed via vacuum annealing of 2H-MoTe2 crystals.20 However, the formations exist in bulk crystals rather than isolated bundles. Finding a new route for controllable synthesis is still challenging, while necessary for future researches and applications. Especially when it is compatible with phase-tuning growth of MoTe2,21-23 the 1D-2D heterostructures and related potential phase-controlled electronics can be demonstrated. In this work, we grow the semi-metallic Mo6Te6-nanowire-phase networks and films using molecular beam epitaxy (MBE). The temperature-dependent growth morphology is rather similar to the recently reported 2D films-1D nanoribbons growth of MoSe2.24-25 Interestingly, 1D Mo6Te6 nanowires are herein observed rather than MoTe2 nanoribbons, through in-situ reflected high energy electron diffraction (RHEED),

scanning

tunneling

microscopy/spectroscopy

(STM/STS),

X-ray

photoelectron spectroscopy (XPS) and ex-situ Raman spectroscopy. The lattice structure of Mo6Te6 NWs is schematically shown in Figure 1a. The unit cell of a single Mo6Te6 nanowire is consisted of two staggered stacks (red triangles as marked in Figure 1a). Each stack is composed of an equilateral Mo3Te3 triangle, with three Te/Mo atoms at its three vertexes/sides. Then the bulk crystal is


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formed by the monoclinic assembly of isolated Mo6Te6 NWs, with the lattice constants of a=1.89 nm and c=0.47 nm. Moreover, every NW is shifted by c/2 along the [001] directions relative to its adjacent NWs. And all NWs are slightly rotated by 6.1˚ through optimizing-structures. The MBE growth is carried out on the highly oriented pyrolytic graphite (HOPG) which ensures excellent van der Waals (vdW) epitaxy. Various growth morphologies are observed with varying the substrate temperature (Tsub). When Tsub is kept at 250 ~ 400 oC, the typical 2D growth is presented in Supporting Information Figure S1a. In-situ RHEED, STM and XPS characterizations confirm MoTe2 films synthesized, among which, a typical atomic-resolution STM image with Moiré pattern is shown in Figure 1b, indicating 2H-MoTe2 is grown in this area. These results have been detailedly analyzed in our previous work.21 When Tsub is increased to ~ 450 oC, 1D Mo6Te6 NWs arise and coexist with 2D islands (Figure S1b). Moreover, the three-phase coexistence of semiconducting 2H-phase MoTe2, semi-metallic 1T'-phase MoTe2 and semi-metallic Mo6Te6 NWs could be formed (Figure 1c), fabricating 1D-2D vertical heterostructures with atomically sharp steps (Figure 1d). 1D growth becomes dominant (Figure S1c) as Tsub increasing to ~500 oC. The RHEED patterns of both 2H- and 1T'-MoTe2 disappear, while streaky patterns of Mo6Te6 networks arise. The lattice distance is calculated as 0.47 nm, which is consistent with the period length along the [001] directions. The height profile in Figure S1c inset shows 1.0 nm, 1.8 nm and 2.6 nm edge steps, corresponding to height of monolayer, bilayer and trilayer Mo6Te6 NWs. It is worth noting that the sub-micrometer-length trilayer


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Mo6Te6-NW-bundles are in perfect alignment, exactly standing along the 0.33 nm graphene steps. However, NWs grown on the terraces are not as straight, with dense bent and branch structures (Figure S2a-b). Such structures7, 19 mostly arise with Te atoms missing, suggesting that the grown Mo6Te6 NWs are slightly less stoichiometric. As Tsub gets even higher to ~550 oC, the average length of Mo6Te6 NWs becomes much shorter, while the density of bent and branch structures gets even higher (Figure S1d). We attribute this to more Te vacancies formed at higher growth temperature. The corresponding temperature-dependent average length from STM statistics and Te/Mo ratio from XPS analysis are summarized in Figure S1e, suggesting Tsub ≈ 500 o

C is most suitable to synthesize long and stoichiometric Mo6Te6 NW bundles. In Figure 1e, further zoomed-in STM image on Mo6Te6 NWs shows upper two

Te-layers are both probed, that is to say, the topmost three Te atoms in every unit cell of every single nanowire (as marked blue in the inset model) are simultaneously resolved. Through line scans, we can measure the period length along their [100] and [001] directions to be 1.82 nm and 0.46 nm, respectively, close to the lattice constants listed above. Moreover, the c/2 shift along the [001] directions between two adjacent NWs is observed. All these structure properties are consistent with the model of Mo6Te6 nanowires, rather than 2H- or 1T'-MoTe2 nanoribbons (Figure S2c-d). To further compare the electronic structures of 2D MoTe2 and 1D Mo6Te6, scanning tunneling spectroscopy measurements combined with density functional theory (DFT) calculations are performed. Figure 2a presents the DFT-calculated bandstructure, density of states (DOS), as well as the experimental STS spectra of


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2H-MoTe2, all confirming its semiconducting characteristic. The differential conductive dI/dV curve, which is acquired on the marked point in Figure 1b, exhibits typical “U”-shape, consistent with the calculated DOS. Moreover, from STS spectra we can derives an energy gap of ~1.09 eV, with the valence band maximum (VBM) at 0.83 eV below the Fermi level (see inset of Figure 2a). This value is in agreement with our calculated bandgap, and also close to the reported experimental works.22, 26-27 For 1D Mo6Te6 NWs, however, the electronic property is extremely distinct. The STS measurement at the point in Figure 1e over a wide energy range (black curve in Figure 2b) shows a pretty low local density of state (LDOS) at the range of 0~1 eV above the Fermi level (EF). As we further approach the tip and narrow the energy range (blue curve) at exactly the same position, a nearly “V”-shape dI/dV curve with a negligibly small energy gap around EF is acquired, which reveals more of a semi-metallic characteristic. Such results are experimentally reproducible on different locations and different samples (Figure S3), showing 1D Mo6Te6 has an obviously different electronic structure from 2H-MoTe2. More interestingly, this STS feature is not identical with the former experimental work on metallic Mo6Se6 NWs,13 which have the similar atomic structures, nevertheless the intrinsically different electronic structures. According to the reported theoretical work, the isolated single-wire Mo6S6 and Mo6Se6 are metallic, with three bands crossing EF.5,

18

However, when the S and Se atoms are replaced by less

electronegative Te, the direction of charge transfer between transition metal and chalcogen atoms is reversed. As a consequence, the isolated single-wire Mo6Te6


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becomes an indirect bandgap semiconductor,5 which is also consistent with our own calculated results (Figure S4). Furthermore, the Mo6Te6 bulk in a monoclinic assembly shows a partially occupied conduction band, determining the metallic nature.20 Experimentally, however, neither an isolated single-wire Mo6Te6 nor the Mo6Te6 bulk is commonly observed in our MBE-grown samples. Instead, Mo6Te6 NWs mostly exist in lateral multiple bundles with a typical width distribution between 4 to 8 wires (see in Figure S5). Regarding this, we perform DFT calculations based on a lateral 4-wire Mo6Te6 model and present the bandstructure and DOS in Figure 2b. An ultra-small bandgap ( directions, as illustrated in Figure 4e. STM images of similar structures are also shown in Figure S6. Therefore, we demonstrate that the Mo6Te6 NWs are formed along the zigzag edges of 2H-MoTe2. A similar model has been presented in reported STEM researches.20 STS mapping along the black line in Figure 4d is further performed. As is shown in Figure 4f, an obvious boundary in dI/dV color mapping exists at Curve IV, which is exactly taken at the 2H-MoTe2/Mo6Te6 interface. On its left, Curve I~III exhibit typical semiconducting characteristics, which agree with 2H-MoTe2; while from Curve V on, the STS spectra transit into semi-metallic properties, which belong to Mo6Te6 NWs. Moreover, at the junction of semiconductor-semimetal, an upward band bending of ~0.4 eV is observed on 2H-MoTe2, which might be attributed to hole injection from Mo6Te6 NWs. As we further anneal the sample to 600 oC for 1 h, the complete phase transition from MoTe2 to Mo6Te6 occurs. Different from the direct MBE-grown sample, Mo6Te6-nanowire-phase ultrathin films (Figure 4c) rather than networks (Figure S1c) are formed. The uniform thickness of 1.0 nm and 1.8 nm correspond to monolayer


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and bilayer Mo6Te6-nanowire-phase films, respectively. The terraces-and-steps morphology is almost maintained while annealing, however, the film coverage is measured as 0.72, decreasing by ~ 30% compared to MoTe2. We attribute it to the average surface area of each Mo unit decreasing from 0.11 nm2 (MoTe2) towards 0.07 nm2 (Mo6Te6). In other words, nearly no Mo atoms desorb or re-evaporate from the substrate while annealing. Zoomed-in streak-line morphology is presented in Figure S7a-b, with the three-fold orientation remaining. The typical scale of nanowire bundles is several tens of nanometers, which is mainly limited by the original MoTe2 island sizes. Moreover, two typical defect types are commonly observed. In Figure S7c, the topmost Te atoms are missing in the marked nanowire, resulting in the height 0.1 nm lower than the adjacent lines. Such line defects are rather dense, as is shown in Figure S7a. While in Figure S7d, a point defect is marked by a black arrow. A branch structure is also found in Figure S7d, which arises from the lattice distortion at the joint point. Similar branch structures have been discovered and discussed in Mo6Se6 nanowires.19 It is worth noting that all these line and point defects are Te-loss-type. As a consequence, the Te/Mo ratio obtained from XPS spectra (Figure S8) is 0.77, showing less stoichiometric properties in this sample. Compared to the direct-grown method, the two-step approach demands a higher growth temperature, leading to a higher evaporation rate of Te atoms, therefore forming denser Te vacancies at surfaces. In conclusion, controllable MBE growth of novel 1D semi-metallic Mo6Te6 NWs has been achieved. In-situ RHEED, STM/STS, XPS and ex-situ Raman spectroscopy


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characterizations affirm synthesis of Mo6Te6 nanowires rather than MoTe2 nanoribbons. Through direct growth at 500 oC, Mo6Te6-nanowire-phase networks are obtained with nearly Volmer-Weber growth mode; while through a two-step growth approach (350 oC growth and 600 oC annealing), uniform Mo6Te6-nanowire-phase ultrathin films with terraces-and-steps morphology are obtained. Moreover, such semi-metallic Mo6Te6-nanowire-phase networks and films can be integrated into semiconducting MoTe2 films via temperature-dependent growth, forming novel 1D-2D heterostructures with atomically sharp interfaces. Such results would suggest potential applications in future phase-controlled electronic and optoelectronic devices.

METHODS Sample Growth: MBE growth was performed in a commercial SPECS ultra-high vacuum (UHV) MBE system with a base pressure of 1×10-10 mbar. The HOPG substrates ware freshly exfoliated in air and degassed in UHV chambers, and the graphene-terminated 6H-SiC(0001) substrates ware prepared by flash heating to 1400 oC.30-31 The substrate temperature was controlled by heating current of filament radiation and calibrated with an infrared pyrometer. High-purity elemental Mo (99.95%) and Te (99.9999%) were simultaneously evaporated from an electron beam evaporator and a Knudsen effusion cell, respectively. To assure stoichiometric samples grown, a high Te/Mo flux ratio of >10 was used, while the substrate temperature was kept between Te and Mo source temperatures. All MBE growth was followed by 5 min annealing with growth temperature maintained and all evaporators


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shut off. The growth process was monitored by a real-time RHEED system operated at 25 kV. Sample Characterizations: After growth, samples were transferred to the adjacent UHV chambers for in-situ characterizations. All STM/STS investigations were performed at room temperature (RT) using a polycrystalline Pt/Ir tip. STM images were acquired in constant current mode and processed by Nanotec WSxM software,32 while STS data were taken at a constant tip-sample distance by shutting off the feedback controller. XPS spectra were in-situ obtained at RT by a PHOIBOS 100 hemispherical energy analyzer with X-ray source (Al Kα 1486.6 eV) operated at 200 W. Ex-situ Raman spectroscopy was performed after sample transferred to the ambient air. The spectra were recorded using a WITec Alpha 300R confocal spectrometer in the backscattering configuration with a 50× objective, 1800 lines/mm grating and 532 nm excitation laser. First-Principle Calculations: The calculations were carried out with the first-principle based on the density functional theory (DFT).33-34 The exchange correlation energy was approximated by the generalized gradient approximations (GGA)35 with the functional Perdew, Burke, and Ernzerhof (PBE).36 The MoTe2 was simulated using a 2 × 3 supercell along the x- and y- directions as the periodic boundary conditions. The Mo6Te6 nanowires were treated by supercell geometry using the z- direction as the periodic boundary. The other directions beyond the periodic boundary, were kept more than 10 Å inter-distance to avoid interactions. The Brillouin zones were sampled by a 11 × 11 × 1 mesh and 1 × 1 × 11 mesh,


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respectively. A mesh cutoff of 125 Hartree was chosen. The stress of primitive cell boundary was converged to less than 0.01 eV/Å2 and the maximum force on each atom was less than 0.01 eV/Å. All calculations were implemented by the Atomistix ToolKit (ATK) packages.

ASSOCIATED CONTENT Supporting Information Available: Additional figures including STM images, XPS spectra and related analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGEMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (No. 11574395), the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics (No. KF201411), the Open Foundation of State Key Laboratory of High Performance Computing (No. 201301-02), and the research project of National University of Defense Technology (No. JC15-02-01).


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(31) Wang, Q.; Zhang, W.; Wang, L.; He, K.; Ma, X.; Xue, Q. Large-scale Uniform Bilayer Graphene Prepared by Vacuum Graphitization of 6H-SiC (0001) Substrates. J. Phys.: Condens. Matter 2013, 25, 095002. (32) Horcas, I.; Fernández, R.; Gomez-Rodriguez, J.; Colchero, J.; Gómez-Herrero, J.; Baro, A. WSXM: a Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (33) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864. (34) Kohn, W.; Sham, L. J. Self-consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (36) Xu, X.; Goddard III, W. A. The Extended Perdew-Burke-Ernzerhof Functional with Improved Accuracy for Thermodynamic and Electronic Properties of Molecular Systems. J. Chem. Phys. 2004, 121, 4068-4082.


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FIGURE CAPTIONS

Figure 1. STM images of 2D MoTe2 - 1D Mo6Te6 heterostructures grown at 450 oC on HOPG substrates. (a) Lattice structures of Mo6Te6 NWs in ball-and-stick model, green balls represent Mo atoms and orange balls represent Te atoms; (b) Atomic-resolution STM image of 2H-MoTe2 (Vb=0.5V, It=500pA); (c) Areas with three-phase coexistence (Vb=-0.5V, It=100pA); (d) Atomically sharp interfaces of 2H-MoTe2 and Mo6Te6 vertical heterostructures (Vb=0.5V, It=500pA); (e) STM image of 1D Mo6Te6 NW bundles with topmost three Te atoms (blue) resolved in every unit cell (Vb=0.5V, It=500pA). Black arrows in (b) and (e) indicate the points where STS spectra are acquired.

Figure 2.

Spectroscopic characterizations. DFT-calculated bandstructures, DOS and

in-situ STS spectra for (a) 2H-MoTe2 and (b) Mo6Te6 NWs. The blue and black curves in (b) dI/dV spectra are measured over different energy range while at the same position. (c) In-situ XPS spectra and (d) Ex-situ Raman spectra of MBE-grown MoTe2 films (black curves) and Mo6Te6 NWs (red curves) on HOPG substrates.

Figure 3. Time-dependent STM images of Mo6Te6 NW networks grown on graphene/SiC(0001) substrates. (a) Morphology after 5 min growth; (b) Zoomed-in STM images of (a); (c) Morphology after 10 min growth; (d) Morphology after 30 min growth. STM parameters: Vb=1V, It=100pA.


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Figure 4. Mo6Te6-nanowire-phase ultrathin films grown on HOPG substrates. (a) MoTe2 monolayer grown at 350 oC; (b) Sample annealed to 550 oC；(c) Sample further annealed to 600 oC; (d) Atomic-resolution STM image and (e) Schematic diagram of 2D MoTe2 - 1D Mo6Te6 lateral heterostructures; (f) 2D STS color mapping and representative STS curves taken along the black line in (d). STM parameters: Vb=1V, It=100pA.


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Figure 1. STM images of 2D MoTe2 - 1D Mo6Te6 heterostructures grown at 450 oC on HOPG substrates. (a) Lattice structures of Mo6Te6 NWs in ball-and-stick model, green balls represent Mo atoms and orange balls represent Te atoms; (b) Atomic-resolution STM image of 2H-MoTe2 (Vb=0.5V, It=500pA); (c) Areas with three-phase coexistence (Vb=-0.5V, It=100pA); (d) Atomically sharp interfaces of 2H-MoTe2 and Mo6Te6 vertical heterostructures (Vb=0.5V, It=500pA); (e) STM image of 1D Mo6Te6 NW bundles with topmost three Te atoms (blue) resolved in every unit cell (Vb=0.5V, It=500pA). Black arrows in (b) and (e) indicate the points where STS spectra are acquired. 369x228mm (119 x 119 DPI)


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Figure 2. Spectroscopic characterizations. DFT-calculated bandstructures, DOS and in-situ STS spectra for (a) 2H-MoTe2 and (b) Mo6Te6 NWs. The blue and black curves in (b) dI/dV spectra are measured over different energy range while at the same position. (c) In-situ XPS spectra and (d) Ex-situ Raman spectra of MBE-grown MoTe2 films (black curves) and Mo6Te6 NWs (red curves) on HOPG substrates. 440x297mm (119 x 119 DPI)


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Figure 3. Time-dependent STM images of Mo6Te6 NW networks grown on graphene/SiC(0001) substrates. (a) Morphology after 5 min growth; (b) Zoomed-in STM images of (a); (c) Morphology after 10 min growth; (d) Morphology after 30 min growth. STM parameters: Vb=1V, It=100pA. 238x219mm (119 x 119 DPI)


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Figure 4. Mo6Te6-nanowire-phase ultrathin films grown on HOPG substrates. (a) MoTe2 monolayer grown at 350 oC; (b) Sample annealed to 550 oC；(c) Sample further annealed to 600 oC; (d) Atomic-resolution STM image and (e) Schematic diagram of 2D MoTe2 - 1D Mo6Te6 lateral heterostructures; (f) 2D STS color mapping and representative STS curves taken along the black line in (d). STM parameters: Vb=1V, It=100pA. 375x225mm (119 x 119 DPI)


Phase-Controlled Growth of One-Dimensional Mo6Te6 Nanowires

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