Ultra-narrow WS₂ nanoribbons encapsulated in carbon nanotubes

Abstract

Layered tungsten disulfide nanostructures are of both fundamental and technological interest. The widths of currently synthesized WS₂ ribbons are in the microscale. By using single-walled carbon nanotubes and double-walled carbon nanotubes as templates, we fabricate WS₂ nanoribbons with smooth zigzag edges and uniform widths down to 1–3 nm and layer numbers down to 1–3, dependent on the nanotube diameter. Although bulk WS₂ is a nonmagnetic semiconductor, the ultra-narrow free-standing zigzag-edged WS₂ nanoribbons turn out to be magnetic or nonmagnetic metals depending on the edge passivation way according to our first-principles calculations, whereas the ultra-narrow armchair-edged WS₂ nanoribbons remain nonmagnetic semiconductors with a narrow gap.

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Introduction

Layered transition-metal dichalcogenide nanostructures (WS₂, MoS₂, etc.) are attracting much attention due to fundamental and technological interest.¹WS₂ nanostructures have been applied in lithium ion batteries,² lubricants,³ tips of scanning probes,⁴ and visible-light-sensitivity phototransistors.⁵ Compared with bulk WS₂, nanostructured WS₂ has new properties because of quantum confinement and edge effects. Similar to carbon nanotubes and fullerenes, WS₂ can also form nanotubes^6–10 and fullerene-like structures,^10–12 which could be regarded as a result of the rolling of the planar WS₂ layers. Restriction of two-dimensional materials in one direction generates nanoribbon structures. Particularly, graphene nanoribbons (GNRs) have become the focus of nanoscience in recent years.^13–17 The properties of GNRs are found to be width- and edge-dependent.¹⁸ Different from conducting graphene, GNRs of sub-10 nm width are always semiconducting.¹⁶WS₂ ribbons have also been synthesized but their widths are in micro- or milliscale.^19–21 In-plane quantum confinement effects and edge effects are expected to be insignificant in this size range. So far, the synthesis of ultra-narrow single-layer or few-layer WS₂ nanoribbons (WS₂NRs) with a width of less than the exciton Bohr radius of WS₂ (4 nm) has never been reported. Recently, we successfully synthesize ultra-narrow MoS₂ nanoribbons (MoS₂NRs) by using single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) as templates.²² Subsequent first-principles calculations reveal that unlike bulk MoS₂, which is a nonmagnetic semiconductor, the ultra-narrow zigzag-edged MoS₂NRs always have a magnetic ground state with electronic types ranging from metal, semiconductor, and even to half-metal.^22,23 Quantum transport calculations demonstrate that the finite MoS₂NRs have high spin filter efficiency when coupled to Li electrodes, which suggests a conceptually new application of MoS₂ nanostructures in spintronics.²²

In this paper, we report the synthesis of ultra-narrow WS₂ nanoribbons inside carbon nanotubes, following the same procedure to synthesize ultra-narrow MoS₂ nanoribbons. The synthesized WS₂NRs have smooth edges, and even have widths down to 1–3 nm, and layer numbers down to 1–3 layers, depending on the inner diameter of the carbon nanotube. In contrast to nonmagnetic semiconducting bulk WS₂ and magnetic semiconducting or metallic ultra-narrow zigzag-edged MoS₂NRs, first-principles calculations demonstrate that the ultra-narrow zigzag-edged WS₂NRs are always metallic and can be magnetic or nomagnetic, dependent on the passivation method.

Results and discussion

Formation of WS₂ was confirmed by Raman scattering measurements. Remarkable Raman signals (E_2g¹ mode, 350 cm⁻¹ and A_1g mode, 416 cm⁻¹)²⁴ characteristic of WS₂ are observed for WS₂@SWCNTs and WS₂@DWCNTs, as shown in Fig. 1. The intensive signals at 148–162 cm⁻¹ originate from the radial breathing mode (RBM) of SWCNTs. Fig. 2a shows an annular dark-field image of single-layer WS₂NRs, in which the bright spots correspond to W atoms. The uniform width and smooth edges of the WS₂NRs can be clearly seen from the image. Micro energy-dispersive X-ray (EDX) analysis performed on the area denoted by a rectangle corroborates the presence of W and S elements (Fig. 2b).

Fig. 1 Raman spectra of SWCNT, WS₂@SWCNT, and WS₂@DWCNT. The E_2g¹ and A_1g modes are from WS₂NRs.

Fig. 2 (a) Annular dark-field image of WS₂NRs in SWCNTs. The identification of nanoribbons is based on the fact that the width of this encapsulated quasi-one-dimensional structure is obviously larger than the height (for example, see Fig. 5c). We found that all the nanoribbons inside SWCNTs consist of one WS₂ layer and the formation of multi-layer WS₂NRs only occurs in DWCNTs with larger diameters. Thus the crystals in this dark-field image are ascribed to single-layer WS₂NRs rather than multi-layer WS₂NRs or nanowires. (b) Micro EDX signals obtained from the area denoted by the rectangle in (a), confirming the presence of W and S elements. Signals of Cu come from the supporting foil.

The diameter of SWCNTs used for encapsulation of WS₂NRs is mainly in the range of 1.3–1.6 nm. Only single-layer WS₂NRs are observed inside SWCNTs. Fig. 3a and 3b show the high-resolution transmission electron microscopy (HR-TEM) images of encapsulated single-layer WS₂NRs that are parallel to the incident electron beam. The thick lines correspond to the WS₂NRs (denoted by arrows), and the adjacent thinner lines are the nanotube walls. Fig. 3c shows a structural model of WS₂NRs encapsulated inside SWCNT. A WS₂ layer consists of three layers of atoms, i.e., a layer of W atoms in the center and two layers of S atoms at two sides. The dark lines denoted by arrows in HR-TEM images actually represent three layers of atoms, however the W and S layers are not distinguishable. When using DWCNTs with inner diameters of 1.5–4.0 nm as templates, double-layer WS₂NRs are generated. Typical HR-TEM images are shown in Fig. 3d and 3e, in which the two parallel dark lines correspond to the WS₂ layers. The distance between the two WS₂ layers is 0.62 nm, in agreement with that in hexagonal phase of WS₂ (0.618 nm). Besides double-layer WS₂NRs, single-layer and triple-layer WS₂NRs encapsulated in DWCNTs are occasionally observed (see ESI, Fig. S1†). The lengths of WS₂NRs inside SWCNTs and DWCNTs are usually tens of to over one hundred nanometres.

Fig. 3 (a) HR-TEM image of a single-layer WS₂NR (denoted with an arrow) encapsulated in SWCNT. (b) Two adjacent SWCNTs filled with single-layer WS₂NRs. (c) Schematic model of a single-layer WS₂NR encapsulated in SWCNT. (d, e) HR-TEM images of double-layer WS₂NRs encapsulated in DWCNTs. (f) Schematic model of a double-layer WS₂NR encapsulated in DWCNT. In model, blue ball: W; yellow ball: S; grey ball: C.

When the WS₂NR basal plane is nearly perpendicular to the incident electron beam, the structure of the WS₂NR can be resolved at atomic level by HR-TEM. Fig. 4a shows a WS₂NR with a width of ∼1.5 nm. From the hexagonal pattern in the image, the WS₂NR is unambiguously assigned to a zigzag-edged nanoribbon. Simulation of single-layer WS₂NR with different tilting angles under an electron beam (Fig. 4b and Fig. S2†) reveals that the position of W atoms is darker than that of S atoms, and the contrast difference becomes more notable with increasing tilting angle x and y. Accordingly, the dark spots in Fig. 4a are ascribed to W atoms, as illustrated in Fig. 4c, and the position of S atoms are then identified. Following the convention of GNRs,¹⁸ we use N-ZWS₂NR (N-AWS₂NR) to denote a zigzag-edged and (armchair-edged) WS₂NR with N zigzag chains (dimer lines) across the ribbon width. The WS₂NR in Fig. 4a is identified as a 6-ZWS₂NR. The distance between the adjacent W atoms in Fig. 4a is 0.32 nm, which is very close to the theoretical distance of 0.315 nm. From the annular dark-field image in Fig. 2a, we can see three WS₂NRs. According to the arrangement manner of the W atoms, the WS₂NR in the top-left corner of the image is identified as a 5-ZWS₂NR and the other two nanoribbons are identified as 4-ZWS₂NRs with width of 1.0 nm, which is the smallest width measured for the WS₂NRs in our sample. In fact, all the plane views of the WS₂NRs in our sample have zigzag edges, indicating that zigzag-edged WS₂NRs are more stable than armchair-edged ones.

Fig. 4 (a) Atomic-resolved HR-TEM image of a single-layer WS₂NR encapsulated in a SWCNT. The yellow hexagons help with the identification of the atom structure, and this WS₂NR is identified as 6-ZWS₂NR. (b) HR-TEM simulation and structural model of a single-layer WS₂NR at 4 different tilting angles. The tilting angle is x = 0° and y = 0° when the WS₂NR plane is perpendicular to the incident electron beam. More simulated images can be found in the ESI.† (c) An enlarged view of the image in (a). The dark spots correspond to W atoms. After assignment of the W atoms, the position of the S atoms are identified. (d) Schematic model of the WS₂NR in (a). In the model, blue ball: W; yellow ball: S.

Besides WS₂NRs parallel or perpendicular to the electron beam, WS₂NRs in other orientations are also observed. Fig. 5a shows a WS₂NR with a tilting angle of 0° < x < 90° and 0° < y < 90°. Fig. 5b is the structural model. Due to the confinement of carbon nanotubes, the width of the encapsulated WS₂NRs is close to the inner diameter of the carbon nanotubes. However, the accurate determination of the width of WS₂NRs are sometimes difficult because of the inclination of the nanoribbons. The measured maximum width of WS₂NRs is ∼3 nm in our experiments.

Fig. 5 (a) HR-TEM image and (b) schematic model of a tilted WS₂NR encapsulated in DWCNT. (c) HR-TEM image showing a twisted single-layer WS₂NR encapsulated in SWCNT. In the schematic model, blue ball: W; yellow ball: S; grey ball: C.

WS₂NRs are often found to be twisted inside carbon nanotubes, and the host carbon nanotubes are radially deformed to a large extent. One example is shown in Fig. 5c. The twist of both the nanoribbon and the SWCNT can be clearly seen. As the schematic model shows, the cross section of the SWCNT expands in the direction of the nanoribbon plane as a consequence of the strong interaction between the nanoribbon edge and the carbon nanotube wall, and meanwhile, shrinkage of the cross section in the perpendicular direction occurs. The minoraxis of the SWCNT cross section is measured to be 1.0 nm in Fig. 5c. Based on measurements of many WS₂NR@SWCNTs, the minoraxis of the distorted SWCNT cross section is in the range of 1.0–1.1 nm and the distance between the basal plane of the WS₂ layer and the nanotube wall is 0.50–0.55 nm. Compared with the initial diameter range of 1.3–1.6 nm, it is apparent the SWCNTs have undergone severe radial deformation. For DWCNTs filled with double-layer WS₂ nanoribbons, the distance between the basal plane of the WS₂ layer and the innertube wall is also found to be 0.50–0.55 nm. These results indicate that a stable hybrid structure is formed when the distance between the nanoribbon and the nanotube gets into this range and the radial deformation of the nanotubes tends to satisfy this rule in the formation process of the nanoribbons.

We perform an intensive DFT calculation for the free-standing 1 nm wide 4-ZWS₂NRs and 7-AWS₂NR to identify the details of the edge and explain why only the zigzag edge is observed. 0%, 50% and 100% S-saturated W edges and 100% and 50% saturated S edges are considered in our 4-ZWS₂NR model. We use a symbol of W_p%S-S_q% to denote the edge configuration of the 4-ZWS₂NR with a p% S-saturated W edge and a q% saturated S edge. Only the intact 7-AWS₂NR model is considered in this paper. Supercells containing two and one unit cells are built for the 4-ZWS₂NRs and 7-AWS₂NR, respectively.

The optimized structures of the WS₂NRs are shown in Fig. 6a–6g. The triple-layer networks are kept at both the 4-ZWS₂NRs and 7-AWS₂NR after full relaxation, but the thickness of the marginal triple-layer changed by −0.18 Å to 0.16 Å, comparing to the bulk and single-layer WS₂, whereas the central triple-layer was nearly unchanged. The fully bare and fully S-saturated W edges of the 4-ZWS₂NRs shift inward notably by 0.16–0.21 Å, while the half S-saturated W edges shift outward slightly by 0.01–0.05 Å. The fully and half-saturated S edges shift outward by 0.01–0.06 Å and 0.05–0.09 Å, respectively. The neighboring S atoms of the half-saturated S edges significantly approach each other in the vertical direction by 1.24–1.35 Å. We find that the W atoms at the half S-saturated W edge are dimerized with an amplitude of 0.11–0.54 Å. Similar phenomenon was found in their structural analogue ZMoS₂NRs.²² The edge W atoms of the 7-AWS₂NR shift inward by 0.39 Å.

Fig. 6 (a–g) Optimized structures of the 4-ZWS₂NRs with different edge configurations (see text) and the 7-AWS₂NR. (h–k) Ground-state spatial spin density distributions arranged by descending δG of the 4-ZWS₂NRs. The nonmagnetic 4-ZWS₂NRs with W_(100%S)–S_(50%) and W_(50%S)–S_(50%) edge configurations are not shown here. The parallel and antiparallel spin orientations between the two edges in (i) are degenerate in total energy. Red and green are used to denote the spin-up and spin-down orientations, respectively. The isovalue is 0.01 a.u. Blue ball: W; yellow ball: S.

Direct comparison of cohesive energy per atom of WS₂NRs is not a suitable way because the relative stability depends upon the chemical potentials of their constituents, which in turn represent environmental conditions.²⁵ Following previous works,^25–27 we adopt the molar (per atom) Gibbs free energy of formation δG to investigate the relative stability of the WS₂NRs. We define δG for the WS₂NRs as

δG = E_Ribbon + n_Wμ_W + n_Sμ_S

where −E_Ribbon represents the cohesive energy per atom of the WS₂NR, n_W and n_S are the molar fraction of W and S atoms for a given structure, and μ_W and μ_S are the chemical potentials of W and S elements at a given state. We choose μ_W and μ_S as the binding energy per atom of bulk W and a crown-like S₈ molecule, respectively. According to the above definition, the lower the δG, the more stable the WS₂NR.

The calculated δG values are given in Table 1. The 4-ZWS₂NRs are more stable than the 7-AWS₂NR with the exception of the 4-ZWS₂NR with the W_(0%S)–S_(50%) edge configuration. The calculated higher relative stability of the 4-ZWS₂NRs over the 7-AWS₂NR explains why the zigzag-edged WS₂NRs are observed. The relative stability of the 4-ZWS₂NRs ascends in the order of: W_(0%S)–S_(50%) < W_(0%S)–S_(100%) < W_(100%S)–S_(50%) < W_(100%S)–S_(100%) < W_(50%S)–S_(50%) < W_(50%S)–S_(100%). The most stable passivation method of 4-ZWS₂NRs is the same as that of 4-ZMoS₂NRs.²² The half S-saturated W edge turns out to be the most stable via comparing δG of the 4-ZWS₂NRs with one identical edge, the fully S-saturated W edge is slightly higher in δG (0.04 eV/atom), and the fully bare W edge is significantly higher in δG (0.17–0.20 eV/atom). This trend is similar to the structural analogue 4-ZMoS₂NRs,²² and we speculate that the half and fully S-saturated W edges probably coexist in sulfidation conditions. The half-saturated S edge is 0.02–0.05 eV/atom higher in δG than the fully saturated S edge.

Table 1 Ground-state molar (per atom) Gibbs free energy (δG) of formation, electronic type, spin coupling between the two edges, total magnetic moment (M_tot), and spin polarization (ξ) of the 1 nm wide WS₂NRs. ΔE is the energy difference per supercell between the magnetic ground state and the nonmagnetic state

Structure	4-ZWS2NRs							7-AWS2NR
	W(0%S)–S(50%)	W(0%S)–S(100%)		W(100%S)–S(50%)	W(100%S)–S(100%)	W(50%S)–S(50%)	W(50%S)–S(100%)
a The parallel and antiparallel spin coupling states between the two edges are degenerate in total energy. b The ground state is nonmagnetic. c M: Metallic. d S: Semiconducting. e A: Antiparallel. f P: Parallel. g one edge is nonmagnetic.
δG/eV	−0.40	−0.45		−0.56	−0.58	−0.60	−0.62	−0.44
ΔE/meV	−27	−86^a		——^b	−53	——^b	−60	——^b
Electronic type	M ^c	M		M	M	M	M	S ^d
Edge Spin coupling	A ^e	P ^f	A	——	——^g	——	——^g	——
M tot (μB)	2.56	3.04	1.28	0.00	1.00	0.00	1.16	0.00
ξ	——	——		——	53%	——	55%	——

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The 4-ZWS₂NRs with W_(0%S)–S_(50%), W_(0%S)–S_(100%), W_(100%S)–S_(100%) and W_(50%S)–S_(100%) edge configurations turn out to be magnetic, while the other two 4-ZWS₂NRs as well as the 7-AWS₂NR are nonmagnetic. As given in Table 1, the magnetic ground states of the 4-ZWS₂NRs are 27–86 meV per supercell lower in total energy than their respective nonmagnetic states. These differences are smaller than the corresponding values of 81–197 meV in 4-ZMoS₂NRs.²² Taken together, the magnetism of the ZWS₂NRs is weaker than that of ZMoS₂NRs. The ground-state spin distributions of the four magnetic 4-ZWS₂NRs are displayed in Fig. 6h–6k. The bare W edge and fully saturated S edge are found to be always ferromagnetic with a localized magnetic moment of M_W(edge) = 1.00–1.16 μ_B and M_S(edge) = 0.26–0.32 μ_B, respectively. Fully and half S-saturated W edges and half-satutared S edges completely quench M_W(edge) and M_S(edge), respectively. The energy differences between the parallel and antiparallel spin orientations between the two edges of the 4-ZWS₂NR with W_(0%S)–S_(100%) edge configuration is less than 1 meV per supercell, implying that the spin coupling between the bare W edge and fully saturated S edge is negligible.

The ground-state band structures for the WS₂NRs are presented in Fig. 7a–7g. The 4-ZWS₂NRs, whether magnetic or not, are all metallic. The orbital wave functions of the two metallic states of the 4-ZWS₂NR with W_(50%S)–S_(100%) edge configuration are analyzed by using a double numerical atomic orbital basis set plus the polarization function (DNP) as implemented in the DMol³ package²⁸ and are found to be chiefly localized on the W and S edges (Fig. 7h–7i). Hence, the spin current travels along the two edges of this nanoribbon. A similar phenomenon was found in the structural analogue 4-ZMoS₂NR.²² The 7-AWS₂NR is a semiconductor with a band gap of 0.39 eV.

Fig. 7 (a–g) Ground-state band structures of the 4-ZWS₂NRs arranged by descending δG and the band structure of the 7-AWS₂NR. Red and blue stand for the spin-up and spin-down states, respectively. The Fermi level is set to zero. (h–i) Isosurfaces of the orbital wave functions of the two conduction bands in the spin-down channel at the Γ point for the 4-ZWS₂NR with W_(50%S)–S_(100%) edge configuration. The isovalue is 0.05 a.u. Red and green are used to indicate the positive and negative signs of the wavefunctions, respectively. Blue ball: W; yellow ball: S.

We perform transport calculations of two 1.9 nm long truncated 4-ZWS₂NRs with W_(100%S)–S_(100%) and W_(50%S)–S_(100%) edge configurations, which are the two most stable magnetic nanoribbons. The former has three and two bands across E_f in the spin-down and spin-up channels, respectively, whereas the latter has two and one bands across E_f in the spin-down and spin-up channels, respectively. The optimized two-probe models with truncated 4-WS₂NRs chemically bonded to a quasi-one-dimensional Li electrodes are shown in Fig. 8. We define the spin polarization ξ of the electron current as

where T^down and T^up represent the transmission coefficients of the spin-down and spin-up channel, respectively. After geometry relaxation of the two-probe system, the 4-WS₂NRs with W_(100%S)–S_(100%) and W_(50%S)–S_(100%) edge configurations show a moderate spin polarization of ξ = 53% and 55% at E_f, respectively, which is suggestive of effective spin filters. These spin polarization values are smaller than those (about 90%) in the finite 4-MoS₂NRs.²² The local density of states (LDOS) at E_f for the two spins are shown in the insets of Fig. 8. The LDOS of down-spin is more extended than that of up-spin, a result in agreement with the larger transmission of down-spin than up-spin at E_f. The high stability and effective spin polarization make the two 4-ZWS₂NRs promising for application in nanospintronics.

Fig. 8 Optimized two-probe models and transmission spectra for the 1.9 nm long truncated 4-ZWS₂NRs with W_(100%S)–S_(0%) and W_(50%S)–S_(0%) edge configurations using the lithium (100) surface as electrodes. Inset plots show surfaces of the constant spin-resolved local DOS evaluated at the Fermi energy. Red and blue stand for the spin-up and spin-down states, respectively. The Fermi level is set to zero. Blue ball: W; yellow ball: S; purple ball: Li.

Finally, we study the properties of the WS₂NR@SWCNT composite. The length of four periods of ZWS₂NR is nearly equal to that of three periods of zigzag SWCNT, and we therefore construct a supercell model consisting of four unit cells of the 4-ZWS₂NR and three unit cells of the zigzag (16,0) SWCNT. Two 4-ZWS₂NRs with W_(0%S)–S_(100%) and W_(50%S)–S_(100%) edge configurations are considered. Fig. 9a and Fig. 10a show the optimized structures of 4-ZWS₂NR_{W(0%S)-S(100%)}@(16,0)SWCNT and 4-ZWS₂NR_{W(50%S)-S(100%)}@(16,0)SWCNT, respectively. The host (16,0) SWCNT is radially deformed with deformation degrees of 40% and 57% measured by aspect ratio. The magnetism of the two inner 4-ZWS₂NRs is completely quenched by the host (16,0) SWCNTs. The 4-ZWS₂NR_{W(0%S)-S(100%)}@(16,0)SWCNT composite is a semimetal and the 4-ZWS₂NR_{W(50%S)-S(100%)}@(16,0)SWCNT composite is a metal (Fig. 9b and 10b). The two deformed host SWCNTs have a small 0.13 eV and zero eV direct band gap (Fig. 9c and 10c, respectively), much smaller than thet band gap of 0.59 eV of pristine (16,0) SWCNTs (Fig. 9b and 10b). The electronic structures of the two 4-ZWS₂NR@(16,0)SWCNT composites are quite similar to those of their 4-ZMoS₂NR@(16,0)SWCNT counterparts.²² The conductivity of the two 4-ZWS₂NR@(16,0)SWCNT composites is mainly contributed to by the inner 4-ZWS₂NRs based on the partial density of states analysis (Fig. 9e and 10e).

Fig. 9 (a) Side and top views of the optimized 4-ZWS₂NR_{W(0%S)-S(100%)}@(16,0)SWCNT composite. (b–d) Band structures of the 4-ZWS₂NR _{W(0%S)-S(100%)}@(16,0)SWCNT, intact (16,0) SWCNT, and deformed host (16,0) SWCNT. (e) Total density of states of the 4-ZWS₂NR_{W(0%S)-S(100%)}@(16,0)SWCNT and partial density of states of its inner bare 4-ZWS₂NR and deformed host (16,0) SWCNT. Blue ball: W; yellow ball: S; gray ball: C.

Fig. 10 (a) Side and top views of the optimized 4-ZWS₂NR_{W(50%S)-S(100%)}@(16,0)SWCNTcomposite. (b–d) Band structures of the 4-ZWS₂NR_{W(50%S)-S(100%)}@(16,0)SWCNT, intact (16,0) SWCNT, and deformed host (16,0) SWCNT. (e) Total density of states of the 4-ZWS₂NR_{W(50%S)-S(100%)}@(16,0)SWCNT and partial density of states of its inner 4-ZWS₂NR with W_(50%S)–S_(100%) edge configuration and deformed host (16,0) SWCNT. Blue ball: W; yellow ball: S; gray ball: C.

Conclusion

Single-layer and few-layer WS₂ nanoribbons encapsulated in SWCNTs and DWCNTs were synthesized via chemical reactions inside the carbon nanotubes. The structure of single-layer WS₂ nanoribbons were resolved at atomic level by using HR-TEM. The encapsulated WS₂ nanoribbons have smooth zigzag edges and uniform widths down to 1–3 nm. The cross section of the host carbon nanotubes is deformed after encapsulation of WS₂ nanoribbons. First-principles calculations demonstrate that the ultra-narrow zigzag-edged WS₂ nanoribbons are more stable than armchair-edged ones, and the electronic and magnetic properties of the WS₂ nanoribbons are dependent on the edge states. The finite magnetic WS₂ nanoribbons have moderate spin polarization from the transport simulation, suggesting novel application concept of WS₂ nanostructures in spintronics.

Experimental

Synthesis and characterization

SWCNTs and DWCNTs were produced and purified as reported previously.^29,30 The purified carbon nanotubes were oxidized in air at 500 °C for 1 h to open the ends of the carbon nanotubes. Then 5 mg of carbon nanotubes were added into 10 mL of saturated aqueous solution of H₃PW₁₂O₄₀. The solution was sonicated for 1 h to disperse the carbon nanotubes and heated under reflux for 2 h to encapsulate H₃PW₁₂O₄₀ into the interior space of carbon nanotubes. Then the carbon nanotubes filled with H₃PW₁₂O₄₀ were collected by filtration and washed with deionized water to remove unencapsulated H₃PW₁₂O₄₀ molecules. WS₂NRs were produced by heating the carbon nanotubes filled with H₃PW₁₂O₄₀ at 800 °C in a H₂S/H₂ atmosphere for 2 h.

HR-TEM: Hitachi H-9000NAR was operated at 100 kV to characterize the structure of the products. JEM-2010F equipped with a CEOS post-specimen spherical aberration corrector (C_s corrector) was operated at 120 kV for atomic resolution imaging. A Gatan 894 CCD camera was used for digital recording of the HR-TEM images. A sequence of HR-TEM images (20 frames) was recorded, with a 0.5 s exposure time for each. After drift compensation, some frames can be superimposed to increase the signal-to-noise (SN) ratio for display. C_s was set in the range 0.5–2 μm, and the HR-TEM images were recorded under a slightly under-focus condition in order to enhance the contrast of the encapsulated materials with respect to the graphene network of carbon nanotubes; this ensured that the contrast of carbon nanotubes could be minimized by fabricating the desirable contrast transfer function (CTF). Image simulations were carried out by using MacTempas software package.

EDX: The JEM-ARM200F incorporating a CEOS pre-specimen spherical aberration corrector (C_s corrector) was operated at 80 kV for EDX analyses. The electron probe, after its aberrations were corrected, featured a current density level higher by an order of magnitude than conventional transmission electron microscopes and was capable of atomic level analysis.

Raman: Jobin Yvon HR-800 spectrometer was used to record the Raman spectra of the samples excited with a laser of 632.8 nm. F_1g mode of silicon at 520.7 cm⁻¹ was used to calibrate the spectra.

Theoretical calculations

Spin-polarized and spin-unpolarized DFT calculations were performed for both the free-standing and composite structures. In the free-standing WS₂NRs model, the vacuums between the adjacent WS₂NRs in the vertical and horizontal directions (y and x directions, respectively) were both kept more than 10 Å, to avoid the interaction between their periodic images. An ultrasoft pseudopotential³¹ plane wave basis set as implemented in CASTEP package³² was used with a cutoff energy of 245 eV. The convergence threshold of geometry optimization was chosen as 0.03 eV Å⁻¹ for the maximum force. On the basis of the equilibrium structures, a larger 300 eV cutoff energy and a 1 × 1 × 49 Monkhorst–Pack³³k-points grid was used to compute the electronic band structures. Spin-polarized quantum transport calculations were performed using the code ATK 2008.10,^34,35 which is based on the DFT coupled with nonequilibrium Green's function (NEGF) method. A mesh cut-off of 150 Ry. and single–ζ basis set (SZ) were employed. In the composite 4-ZWS₂NR@(16,0)SWCNT model, a double numerical atomic orbital basis set (DN) and effective core potentials as implemented in DMol³ package²⁸ were used. Three k-points were applied in the optimizing procedure of all the DFT calculations. Generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) form³⁶ was chosen for the exchange–correlation functional throughout all the calculations.

Acknowledgements

This work was supported by the NSFC (Grant Nos. 90206048, 20771010, 10774003, 90606023, and 20731160012), National 973 Projects (Nos. 2006CB932701, 2007AA03Z311, and 2007CB936200, MOST of China), Fundamental Research Funds for the Central Universities, and Program for New Century Excellent Talents in University of MOE of China. J. L. thank Prof. W. N. Mei for helpful discussion. K. S and Z. L acknowledge the support by CREST and Grant-in-Aid from MEXT (19054017) and Z. L acknowledges the partial support by Hayashi Memorial Foundation for Female Natural Scientists. Dr Okunishi (JEOL) and Dr T. Saito (AIST) are also acknowledged for their kind helps in the EDX and the Raman measurements, respectively.

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Footnotes

† Electronic supplementary information (ESI) available: HR-TEM images of single-layer and triple-layer WS₂NR encapsulated in DWCNTs, HR-TEM simulation of single-layer WS₂NR at ten different tilting angles, and the spin distributions of the 4-ZWS₂NRs. See DOI: 10.1039/c0jm02821e

‡ These authors contributed equally to this work.

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