Nanotubes from the Misfit Layered Compounds MS–TaS2, Where M

May 28, 2014 - Energy of formation of chrysotile nanotubes. A. A. Krasilin , V. V. Gusarov. Russian Journal of General Chemistry 2014 84, 2359-2363 ...
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Nanotubes from the Misfit Layered Compounds MS−TaS2, Where M = Pb, Sn, Sb, or Bi: Synthesis and Study of Their Structure Gal Radovsky,† Ronit Popovitz-Biro,‡ and Reshef Tenne*,† †

Department of Materials and Interfaces, ‡Chemical Research Support, Weizmann Institute of Science, Rehovot 7610001, Israel S Supporting Information *

ABSTRACT: Tubular structures of the MS−TaS2 with (M = Pb, Sn, Sb, Bi) misfit layered compounds are reported. The lattice mismatch between the alternating MS and TaS2 layers leads to a variety of chiral tubular structures. Such tubular structures are studied via scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). For the PbS−TaS2 and SnS−TaS2 tubules, relative in-plane orientations as well as folding vectors of the two subsystems can be determined. However, almost ring-like SAED patterns are obtained for SbS−TaS2 nanotubes precluding exact determination of the relative in plane orientation. Also, very complex diffraction patterns were obtained for BiS− TaS2 nanotubes.

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INTRODUCTION The formation of inorganic nanotubes made of superstructures of MX and TX2 monolayers stacked periodically upon each other is believed to be characteristic for many inorganic misfit layered compounds. Indeed, tubular structures from the misfit compounds SnS−SnS2 and PbS−NbS2 were reported by us in the past. It was shown here that under appropriate synthetic conditions new tubular structures of the MS−TaS2 with (M = Pb, Sn, Sb, Bi) misfit layered compounds can be obtained. The generalization presented here underlines the great importance for the forthcoming research and attempts to expand the family of misfit compound tubular structures and study their unique 1D properties. Misfit Layered Compounds. Misfit layered compounds (MLC) of a general formula (MX)1+y(TX2)m consist of alternating MX and TX2 layers with M = Sn, Pb, Bi, Sb, rare earths; T = Ti, V, Cr, Nb, Ta; X = S, Se: 0.08 < y < 0.28; m = 1−3 were studied previously.1−4 TX2 exhibits pseudohexagonal structure of three atom thick slab in which the metal is surrounded by six chalcogen atoms, either in octahedral or in trigonal prismatic coordination and generally indexed in orthorhombic, monoclinic, or triclinic systems. MX consists of a two atom thick double layer of atoms which can be considered as a distorted NaCl structure with orthorhombic, monoclinic, or even triclinic unit cells. However, for both the TX2 and MX subsystems the monoclinic or triclinic angles do not deviate from 90° by more than several degrees. The value of y is determined by the ratio of the atomic surface densities of a singe layer of TX2 and MX. The interaction between the two layers leads to a mutual incommensurate modulation of the structures of the two subsystems.2−4 Full description of these misfit structures is possible in a (3 + 1) or (3 + 2) dimensional superspace approach.2−4 Here, extra one or two vectors © 2014 American Chemical Society

perpendicular to 3D space are inserted to describe the incommensurate mutual structural modulation between the MX and the TX2 layers. The amount of additional vectors required, depends on the interaction nature and the incommensurate mutual structural modulation between the MX and TX2 layers of different compositions. For example, orthorhombic compounds (MX)1+y(TX2) with (T = Nb, Ta; M = Pb, Sn, most of the rare earths) with only one incommensurate direction, are described in (3 + 1)D superspace approach, whereas the more complex triclinic (SbS)1.15TiS2 is described in a (3 + 2)D superspace approach.5 Often the MX−TX2 stacking stabilizes interface modulated monolayers with structures which are not stable as a bulk phase. This can be exemplified in the case of MX layers like BiS6 and SbS.7,8 Furthermore, Bi and Sb form sulfides in the +3 oxidation state in their well-known Bi2S3 and Sb2S3 compounds. However, their oxidation states in BiS and SbS are not unambiguous.6−8 Others, such as SnS, PbS, and LaS exist as a bulk phase, however their structure is modulated in the (MX)1+yTS2 (T = Nb, Ta) crystal, upon stacking with TX2 and formation of MLC.4,9−11 The formation of otherwise unstable TX2 modulated monolayers can occur also for TX2 compounds which do not exist as a bulk phase, albeit rarely. Examples are TS2 with (T = V, Cr)12 in particular (LaS)1.2CrS24,13 and (PbS)1.12VS2.4,14,15 Also, it was shown that pseudohexagonal VS2 with layered structure can be stabilized by intercalation of Na and that of CrS2 by Na, K, Ag, Cu intercalation.12 However, in most cases when bulk TX2 is stable, its structure varies only slightly upon periodic stacking with MX. Often Received: April 13, 2014 Revised: May 13, 2014 Published: May 28, 2014 3757

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small monoclinic or triclinic distortion are observed in the TX2 lattice when combined, for example, with the SbS layer.7 MLC exhibit incommensurate behavior due to their irrational ratio of the in-plane lattice parameters of the TX2 and MX subsystems at least along one direction. Misfit along one direction occurs for many misfit systems with orthorhombic unit cells including M = Sn, Pb, most of the rare earths; T = Nb, Ta.4,16 It was recently shown that in the ((SnS)1.32)n(SnS2)m system, both SnS and SnS2, almost retain their original bulk structure upon periodic stacking and mutual structural modulation is not believed to occur. Consequently, misfit occurs along both the a and b in-plane directions.17−19 Stability of Misfit Layered Compounds. The stability and the interlayer chemical bonding mechanism of the misfit compounds are not yet fully understood. The effect of incommensurability on their properties has not been studied systematically as yet, either. Charge transfer from the MX to the TX2 layer is believed to be the stabilizing mechanism. Attempts to assess the degree of charge transfer using photoelectron spectroscopy, transport measurements, bond distance by electronic structure calculations, and Raman spectroscopy remain controversial.6−8,20−30 Others attribute the stability of MLC to the presence of weak covalent interlayer bonds between the M and the X atoms of the TX2.24,30 Recently, photoemission microspectroscopy study, suggested that in the case of (PbS)1.13(TaS2), the Ta atoms substitute some Pb atoms in the PbS layer, and Pb atoms substitute Ta atoms in the TaS2 via a so-called metal-cross substitution mechanism.22 This novel metal cross-substitution mechanism alters the charge balance between the two types of layers in a way that strongly enhances the interlayer bonding. However, ab initio electronic structure calculations show that only nonstoichiometric substitution of Pb atoms in PbS by Ta atoms has a stabilizing effect on the misfit lattice.31 Superconductivity. Misfit layered compounds (MS)1+yTS2 where M = Sn, Pb, Bi, rare earth; T = Nb, Ta exhibit a metallic conduction with a superconducting transition at very low temperature T < 3 K.32,33 For M = Sb, similar temperature resistivity dependence was found34 with superconducting transition found later on.35 Superconductivity in 1D has been debated for quite some time. Strictly speaking, superconductivity does not exist in truly 1D structures. However, in most current 1D nanostructures multitude of conducting channels exist. This fact manifests itself through lowering of Tc and the loss of abruptness of the transition, compared to the 3D material.36 Therefore, the comparison between the superconducting transition in lamellar (2D) and tubular (1D) nanostructures of misfit compounds is a subject of great interest. Furthermore, the physics of 1D superconductivity could benefit substantially from the access to almost defect-free 1D nanotubes of the present family of compounds. Nanotubes Formation. MLC are known as potential formers of tubular and rolled up structures. The misfit between the in-plane parameters is believed to be the main driving force for scrolling, which was shown to be further enhanced by the chemical energy saving due to the healing of the dangling bonds at the periphery of the layers.17−19 Upon bending, the differences between the lattice parameters of the two layers are reduced and hence the strain energy is reduced. Also, spontaneous scrolling is mostly expected for asymmetric slabs, that is, with MX on one side and TX2 layer on the other side.17−19,37

The in-plane incommensurate behavior has a large influence on the internal structures of the nanotubes. It is generally assumed that the bending (tubule) axis is perpendicular to the direction along which the lattice parameters differ mostly. In a bulk (PbS) 1 . 1 4 NbS 2 , (SnS) 1 . 1 7 NbS 2 , (PbS) 1 . 1 3 TaS 2 , (SnS)1.16TaS2 MLC,4 the misfit occurs along one in-plane direction a. In this case, the tubule axis is expected to coincide with the fit (commensurate) direction b. However, as was shown previously for PbS−NbS2 tubes19 and will be shown here for PbS−TaS2 and SnS−TaS2 ones, a significant part of the tubes from these compounds do not obey this rule. The lack of a commensurate direction in the a−b plane along which the tubule axis is expected to coincide leads to a higher degree of freedom for the interlayer MX−TX2 orientation and stacking order of the layers along the common c axis, as was previously observed for the SnS−SnS2 system.18 Indeed, the diversity of internal structures manifests itself by the diversity of in-plane orientations between the SnS and SnS2 layers, folding vectors and the stacking periodicities along the common c axis. In this paper, the synthesis of tubular nanocrystals from the PbS−TaS2, SnS−TaS2, SbS−TaS2 and to a lesser extent also the BiS−TaS2 misfit compounds are reported. Although the production yield for the SbS−TaS2 nanotubes was close to 50%, that for the PbS−TaS2 and SnS−TaS2 was ∼5% and that of BiS−TaS2 was merely 1%. Structure of TaS2. The study of the Ta−S phase diagram presents considerable difficulties, due to the large number of intermediate phases close to the composition of TaS2 and the great difficulties in establishing thermodynamic equilibrium.38 Similar to other disulfides of transition-metal elements, tantalum disulfides possess pseudohexagonal lattice. Within a S−Ta−S slab, the Ta atoms are coordinated in trigonal prismatic or in octahedral environment of sulfur atoms depending on whether the two sulfur layers lie one on top of the other or are rotated by 60°.39 The high temperature bulk 1T phase (space group of (P3̅m 1) 38) has octahedral coordination of the Ta atoms (within the slab) with lattice parameters a = 3.36 Å and c = 5.9 Å with one S−Ta−S slab in the unit cell.38,40 This phase can be quenched to room temperature. The room temperature bulk 2H (space group of (P63/mmc)38,41) phase has trigonal prismatic coordination of Ta atoms (within the slab) with lattice parameters of a = 3.315 Å and c = 6.04·2 Å (twice the interlayer spacing) and two S− Ta−S slabs per unit cell.38,40,41 The energy difference between the trigonal prismatic and octahedral coordination is fairly low. Therefore, in other TaS2 polytypes, alternating trigonal and octahedral layers with different stacking orders along the c axis are found.

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EXPERIMENTAL SECTION

Synthesis of the MS−TaS2 tubules with M = Pb, Sn, Sb, and Bi. For the synthesis of the tubular MS−TaS2 (M = Pb, Sn, Sb, Bi) structures, the following molar ratios of the precursors were inserted. For PbS−TaS2 tubes: Ta, PbS, S, at a molar ratio of ∼1:1:1.25. SnS− TaS2 tubes: Ta, SnS2, S, at a molar ratio of ∼1:1:0.5. SbS−TaS2 tubes: Ta, Sb, S at a molar ratio of ∼1:1:3. BiS−TaS2 tubes: Ta, Bi, S, at a molar ratio of ∼1:1:3. In all cases, small amounts of TaCl5 were added to the ampules as a chlorine source, which acts as a transport agent. The ampules were sealed at a vacuum of ∼5 × 10−5 Torr. The hightemperature annealing procedure was carried out in a vertical reactor furnace and involved two steps. In the first step, the ampules were heated at a temperature gradient between 350 °C (bottom part with the precursors) and 800 °C (upper edge) for 1 h. Next, the ampule was moved inside the furnace and subjected to an opposite 3758

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Figure 1. (a) Schematic model of a single layer of TaS2 annotated according to the pseudohexagonal and ortho-pseudohexagonal coordinates, brown and purple unit cells, respectively. Three equivalent “b” directions are indicated by green arrows according to the ortho-pseudohexagonal unit system. (b) Single PbS layer oriented in such a way that its “b” axis is parallel to one of the three equivalent “b” axes of TaS2 according to the orthopseudohexagonal system. Two additional orientations of PbS are also possible. In (a) and (b) the tubule axis is marked by dotted blue line. (The orientation of the TaS2 lattice relative to the tubule axis presented in (a) represents zigzag folding.)

Figure 2. (a) SEM image of a PbS−TaS2 slab being partially scrolled into a conical-tubular crystal. (b) Low and (c) high magnification images of a straight and long PbS−TaS2 tubule with abrupt telescopic step and outer diameter close to 1 μm. High magnification image taken from the area marked by a blue arrow. (d) Part of a tubule (not the one shown in (a)) with apparent wound growing steps.

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temperature gradient, that is, 800 °C (bottom edge with the precursors) and ∼400 °C at the upper edge (except for SnS−TaS2, where the upper edge was kept at ∼300 °C) for 4−8 h. The product accumulated at the upper cold edge of the ampule. Electron Microscopy and Sample Preparation. For transmission electron microscopy (TEM), the product was suspended in ethanol or chloroform and consequently dripped on a lacey carbon/ collodion-coated Cu grids. For scanning electron microscopy (SEM), dry as well as wet methods were utilized. Stubs with Si/Al substrates were prepared in a wet method similarly to TEM. Also, part of the product was placed as a dry powder on a carbon tape without suspension in a solvent. The resulting samples were examined by Philips CM120 TEM, operating at 120 kV equipped with EDS detector (EDAX-Phoenix Microanalyzer) and FEI Tecnai F30-UT high-resolution transmission electron microscope (HRTEM) operating at 300 kV. Zeiss Ultra model V55 SEM and LEO model Supra 55VP SEM equipped with EDS detector (Oxford model INCA) were utilized for these studies, too. X-ray Diffraction. X-ray diffraction (XRD) patterns were obtained with Rigaku TTRAXIII diffractometer (Cu Kα radiation) operating in the Bragg−Brentano (θ−2θ) mode.

RESULTS AND DISCUSSION PbS−TaS2 Tubules. As often occurs in MLC, the MX layer is being modified to adapt to the pseudohexagonal TX2 layer and one of its lattice parameters accommodates a lattice parameter of ∼ √3·a of the TX2 layer, TaS2 in this case. Also, in most cases the TX2 lattice is hardly changed and its structure remains similar to the bulk. Therefore, it is convenient to index the pseudohexagonal (a−b) in-plane lattice parameters TX2 with ortho-pseudohexagonal unit cell with a, b = √3·a as shown in Figure 1a. For the TaS2 layer in a bulk (PbS)1.13TaS2 MLC these in-plane lattice parameters are a = 3.3 Å, b = 5.78 (≈ √3·a) Å. PbS adopts a distorted NaCl double layer structure with a = 5.825 Å, b = 5.78 Å.42 The reported space group of the two PbS and TaS2 layers in the bulk (PbS)1.13TaS2 misfit compound is Fm2m.42 PbS−TaS2 nanotubes are believed to be in many cases isostructural to their PbS−NbS2 counterparts19 as implied from their ED patterns and their chemical similarity. The formation of a concentric tube, most likely, is initiated with the formation 3759

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Figure 3. (a) and (b) TEM images of PbS−TaS2 tubules with PbS and TaS2 layers stacked periodically along the c axis with 1.22 nm periodicity. The folding vectors are different in the (a) and (b) nanotubes. Top: High and low (insets) magnification images. Middle: SAED patterns taken from the area shown in the high-magnification image. Tubule axis is marked by a pink double arrow. Spots pertinent to the same interplanar spacing are marked by segmented rings and their measured values and pertinent Miller indices are indicated (for TaS2, pseudohexagonal labeling system is used). The red circles correspond to TaS2 and the green to PbS. The chiral angles of both TaS2 and PbS are ∼7° in (a) and ∼5.5° in (b). The blue arrows point on the basal reflections produced from the superstructure. See text for the explanation to the small green and red circles. Bottom: Line profile obtained from the region enclosed in the rectangle in the upper image.

(SI)) suggests that the transition from the scroll-like to the concentric tubes is likely to occur by a mechanism involving high-temperature dislocation movement. However, several differences between the Nb and Ta-based nanotubes exist.

of a scroll as shown in Figure 2a. Such a scrolling is driven by the relaxation of the misfit stresses in the a−b plane between the PbS and the TaS2 layers. The existence of dislocation-like defects (as shown in Figure S1 in the Supporting Information 3760

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Figure 4. (a) Schematic model of a single layer of TaS2 annotated according to the pseudohexagonal and ortho-pseudohexagonal coordinates, brown and purple unit cells, respectively, oriented in such a way that its b axis (according to the ortho-pseudohexagonal system) is parallel to the a axis of PbS in (b). In (a) and (b), the tubule axis is marked by dotted blue line. (The orientation of the TaS2 lattice relative to the tubule axis presented in (a) represents armchair folding.)

For example, the maximal outer diameter of the Ta-based tubes can reach in several cases 1 μm as shown in SEM images in Figure 2. Nonetheless, the typical outer diameters of these tubes varies between 150 and 250 nm. TEM analysis of the PbS−TaS2 misfit tubules is shown in Figures 3−5. Figure 3 shows an examples of two nanotubes with PbS and TaS2 layers alternated along their common c axis with lattice periodicity of 1.22−1.23 nm, which stems from the line profiles and basal reflections at the SAED patterns marked by blue small arrows. Following the measurement of the interplanar spacings in the SAED, Miller indices were assigned. The spots with interplanar spacing of 2.87 and 1.65 Å match the interplanar spacings of (10.0) and (11.0) planes of bulk 2H−TaS2 [ICSD col. code 68488],41 whereas that of 2.03 and 4.03 Å match the (220) and (110) planes of interface modulated PbS layer produced in (PbS)1.14NbS2 MLC (ICSD col. code 68701).10 The structure of PbS layer here is believed to be similar to that in PbS−NbS2 misfit compound nanotubes.19 The existence of six equivalent planes for (10.0) and (11.0) in TaS2 should produce six diffraction spots distributed on a circle and equal-azimuthally splintered from each other. However, the diffraction patterns show 12 couples of spots (examples for pairs of individual spots are marked by red small circles in the ED patterns in Figure 3). These 12 couples are equal-azimuthally splintered on a ring-like pattern as marked by the red segmented circles in the ED patterns in Figure 3. The azimuthal angle between each two couples equals 30° as shown in the ED patterns in Figure 3. Such an observation suggests the existence of two types of TaS2 layers with two different folding vectors within the tubule. Also for PbS, 12 couples of 110 and 220 spots are equal-azimuthally distributed on a circle (green-dotted circles). However, because the multiplicity factor (the number of equivalent planes) for these planes is four, this observation suggests three types of layers exhibiting three different folding vectors.

Tubules with eight and four couples of 110 and 220 spots of PbS were also encountered (see SI Figures S2 and S3), indicating the existence of two or only one type of folding vector for the PbS layers within the misfit walls of the nanotubes. As occurs in (MS)1+yTS2 with (T = Nb, Ta, M = Sn, Pb, most of the rare earths) MLC, when TS2 is indexed using ortho-pseudohexagonal coordinates its b axis coincides and is commensurate with the b axis of MS. Also, in PbS layers the angle between the (010) and (110) planes equals ∼44.8°. The normal to (020) planes of PbS coincides with the normal to the (020) planes of TaS2 (the (020) plane of TaS2 is equivalent to the (10.0) according to the pseudohexagonal notation). Therefore, the 020 or 10.0 spots of TaS2 are also expected to form an angle of 44.8° with the 110 or 220 of PbS. Thus, for each out of the 12 (10.0) couples of spots of TaS2, one can always find 110 spot of PbS in azimuthal angle of 44.8°. Examples for that are shown in the ED patterns in Figure 3a and b. Therefore, the in-plane orientation between the TaS2 and PbS can be determined. In Figure 3a two couples of 10.0 spots (020 according to the ortho-pseudohexagonal system) of TaS2 coincide with the tube axis which is also expected to coincide with the 010 and 020 of PbS. However, the 010 reflections of PbS are missing (similarly to the PbS in PbS−NbS2 tubules19 and the 020 reflections can not be distinguished from the 10.0 ones of TaS2. However, the 220 of PbS appear at ∼45° from the tubule axis as expected. Therefore, in this case, the b direction of one type of the TaS2 layers (according to the ortho-pseudohexagonal system) and one type of PbS layer coincide with the tubule axis. Because there are three equivalent “b” directions in TaS2 (according to the ortho-pseudohexagonal system) which are rotated by 60° about the common c axis as marked by the green arrows in Figure 1a there are two additional types of PbS layers (three in total). These types are rotated by 60° relative to each other about the common c axis. Consequently, there are three 3761

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Figure 5. Schematic models show the stacking of the TaS2 and PbS layers along the c axis and their relative in plane orientations for two commonly encountered folding vectors configurations. (a) The structure which stems from the SAED pattern shown in Figure 3a. (b) The structure which stems from the SAED pattern shown in Figure 3b. For every layer head-on view along the c axis is also shown on the right. TaS2 are annotated according to the pseudohexagonal system, and ortho-pseudohexagonal unit cells are shown by purple lines. Ortho-pseudohexagonal axes are shown at the bottom with purple letters on the axes. Three equivalent “b” directions are pointed by green arrows according to the ortho-pseudohexagonal unit system. The uppermost TaS2 layers are rotated by 30° relatively to the bottom ones. Such a rotation is equivalent to rotation by 90° because 90 − 60 = 30° (see text).

quartets of 110 spots of PbS that are azimuthally rotated by 60° from each other, virtually forming 12 pairs of equi-azimuthally splintered couples of spots in the diffraction pattern in Figure 3a. The stacking of the PbS layers is shown schematically in Figure 5a, which shows a model of the entire tubule. Obviously, due to the 6-fold symmetry of the {10.0} planes of TaS2, these two additional PbS layers form equivalent orientation with respect to the adjacent layers of TaS2 as shown in Figure 1a, but their b axis does not coincide with the tubule axis. As for TaS2, in the tube shown in Figure 3a, two types of TaS2 layers with different folding vectors can be recognized. The first one with the 10.0 (020 according to the orthopseudohexagonal system−the b axis) coinciding with the tube axis (“zigzag-like”) was discussed above. The second TaS2 layertype shows additional orientation. Here, the 11.0 is coincident with the tube axis and the b axis deviates by 30° from the tubule axis (“armchair-like”). These orientation variants of the TX2

lattice are expected when the ratio between the in-plane lattice parameters of the MX layer is close to one (i.e bMX/aMX ∼ 1)28, which is 5.78/5.825 = 0.9923 in the PbS layers in (PbS)1.13TaS2 misfit compound.42 Therefore, the mismatch between the a axis (the interplanar spacing of (200) planes) of PbS and (020) of TaS2 (according to ortho-pseudohexagonal system) is very small. In this case the normal to (200) (instead of (020)) of PbS can be parallel to the normal to (10.0) (or (020) according to ortho-pseudohexagonal system) of TaS2. This different orientation between the TaS2 and PbS layers is presented in Figure 4. Here, the second TaS2 layer is rotated by 90° relatively to the first as can be easily noticed by comparing the models in Figures 1a and 4a. However, due to the 6-fold symmetry of (10.0) planes of TaS2 the rotation of the second TaS2 layer relatively to the first about the c axis is 90° − 60° = 30° as shown in the ED in Figures 3a and 3b. This angle equals the one between the (10.0) and (11.0) planes of TaS2 virtually 3762

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Figure 6. (a)−(d) SEM images of an agglomerate of the SnS−TaS2 MLC tubules and partially unrolled SnS−TaS2 sheets. Tubules with different diameters exhibiting apparent scrolling steps are clearly seen.

“making” the tube axis to coincide with the normal to the (11.0) planes (armchair like). The orientation of the TaS2 layer shown in Figure 4a is equivalent to the one shown in Figure 5a (TaS2 second from top). Many of the physical properties, like electrical transport and mechanical are determined to a large extent by the outermost MS−TS2 wall. This makes the knowledge of the orientation of the outermost layer with respect to the tubule axis particularly important. However, the orientation of the outermost layer with respect to the tubule axis can not be determined from the present analysis. The tubule shown in Figure 3b exhibits different folding vectors configuration of the layers compared to the tubule analyzed in Figure 3a. Similarly to the tubule shown in Figure 3a, three types of PbS and two types of TaS2 layers exist and their relative orientation is similar to the tubule described in Figure 3a. This can be easily verified since the azimuthal angle between each two (out of the 12) couples of 10.0 and 11.0 spots of TaS2 equals 30° and the angle between the 110 (or 220) spots of PbS and 10.0 of TaS2 is still ∼45° as in Figure 3a. However, in the tubule presented in Figure 3b none of the b axes of the TaS2 and PbS layers coincide with the tube axis. Instead, the tube axis is normal to one type (out of the three) of the (110) planes of PbS layers. The complete arrangement of the layers in the tubule is shown in Figure 5b. Thus, in contrast to the expectations, Figure 3b shows that the tubule axis does not coincide with any of the three equivalent commensurate b axes of TaS2 and PbS.

The helical arrangement of the TaS2 and PbS layers manifests itself through the difference in the orientation of the atomic lattice on the top and the bottom walls of the tubule. Each of the top and bottom walls of a helical tube will give rise to a splitting of the 11.0, 10.0, 110, and 220 spots of TaS2 and PbS, respectively. In the diffraction patterns shown in Figure 3a and b, all the 12 couples of spots are splintered by an equal angle. The chiral angle can be estimated from the splitting of the mentioned reflections in the diffraction pattern, that is, half the azimuthal splitting angle. Similar angles were obtained for TaS2 and PbS layers which equal 7° for the tube shown in Figure 3a and 5.5° for the tube shown in Figure 3b. Chemical analysis of single PbS−TaS2 nanotubes were carried out by energy dispersive spectroscopy (EDS) and representative spectrum is shown in Figure S4 in the SI. However, the use of EDS here is supportive only and is not expected to be fully quantitative. SnS−TaS2 tubules. Figure 6 shows an agglomerate of the SnS−TaS2 MLC tubules and partially unrolled SnS−TaS2 sheets. The outer diameters of the tubules range between 300 nm and up to ∼1.5 μm in several cases as apparent from Figure 6. However, the typical outer diameters range between 300 and 900 nm. The MLC (SnS)1.16TaS2 has much in common with (PbS)1.13TaS2. Upon stacking with TaS2 and mutual structural modulation, SnS adopts a layered structure that is different from the α-SnS known as Herzenbergite18,43 and is analogous to PbS in (PbS)1.13TaS2 MLC. When indexing in the orthopseudohexagonal system TaS2 exhibits a = 3.316 Å, b = 5.742 3763

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Figure 7. TEM images of SnS−TaS2 tubule with SnS and TaS2 layers stacked periodically along the c axis with 1.24 nm periodicity. (a) High and low (inset) magnification image. (b) line profile obtained from the region enclosed in the rectangle (c) SAED pattern taken from the area shown in the high-magnification image. The tubule axis is marked by a pink double arrow. Spots pertinent to the same interplanar spacing are marked by segmented rings. Their measured interplanar spacings and pertinent Miller indices are indicated (for TaS2, pseudohexagonal labeling system is used). Red segmented circles correspond to TaS2 and green to SnS. Small green circles mark one quartet of 220 reflections indicating one single folding vector for SnS layers which form an angle of 45° with the 10.0 spots of TaS2. The blue arrows point on the basal reflections produced from the superstructure. See text for red and yellow arrows.

(≈ √3a) Å in plane lattice parameters, whereas the distorted rock-salt SnS has a = 5.72 Å, b = 5.742 Å.4 Therefore, in analogy to the (PbS)1.13TaS2 MLC, the misfit occurs along one in plane direction a. In bulk (SnS)1.16TaS2 MLC, TaS2 and SnS have a space groups of Fm2m and Cm2m respectively.4 The diffraction patterns obtained from many SnS−TaS2 tubules have much in common with the ones obtained from their PbS−TaS2 counterparts. Figure 7 shows an example of a SnS−TaS2 tubule (diffraction pattern shown in Figure 7c) with internal structure analogous to some extent to the PbS−TaS2 one shown in Figure S2a in the SI. The diffraction pattern in Figure 7c shows four 110 and 220 couples of SnS spots (marked by small green circles/ellipses). This number (four) equals the multiplicity factor of these planes in this case. Therefore, all SnS layers in this tube have similar orientation about the common c axis. Also, two couples (out of the quartet) of the 110 spots coincide with tube axis, meaning that the b axis (the normal to the (010) planes) deviates by 45° from the tubule axis for every SnS layer in this tube. As for TaS2, 12 couples of each 10.0 and 11.0 spots equal-azimuthally distributed on a ring-like pattern are observed. The number

of 12 doubles the multiplicity factor of 6 for these planes. Therefore, the two types of TaS2 layers are rotated by 30° with respect to each other about the common c axis with two different folding vectors relative to the tubule axis. The folding vectors of the TaS2 layers here and the relative orientation between the TaS2 and the SnS layers are believed to be similar to the ones shown in Figure 3b and Figures S2a and S2b in the SI. This is not surprising because interface-modulated SnS layers are quite isostructural to their PbS counterparts when stacked with TaS2 layers.4 Note that the chiral splitting of the four couples of the both 10.0 and 11.0 spots of TaS2 (out of the 12) can not be clearly seen and appears as a continuous streaks. These spots are closest to the tubule axis and marked by short red arrows (compare with the diffraction shown in Figure S2a in the SI). Also, additional streak appears near the 10.0 spots (marked by yellow arrows) mentioned above. The reason for that is currently not understood. Much of the SnS−TaS2 tubes reported here had a scroll like character (as mainly seen in the SEM images in Figure 6) with a ring-like pattern of the diffraction spots as shown in Figure S5 3764

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Figure 8. (a)−(f) SEM images of a hedgehog-like agglomerates of the SbS−TaS2 misfit compound tubular crystals. The inset in (a) is a low magnification image of a several hedgehog-like agglomerates. (b)−(d) show the range of tubules’s thicknesses. The inset in (d) shows a high magnification of relatively thick tubule (marked by red circle) with very small internal diameter. (e) Images of different tubes; from top to bottom: thick tubule with a high inner/outer diameters ratio; thick tubule with a very small inner/outer diameters ratio; thin nanotube with inner/outer diameters ratio of ∼1/3. In all cases, outer scrolling steps are clearly apparent. (f) Partially unfolded misfit nanosheets forming relatively small and bigger (inset) diameter tubular structures.

in the SI. Such patterns suggest a turbostratic disorder of the layers about the common c axis. However, one can not unambiguously attribute such ring-like patterns to a scroll nature of the tubule, since concentric tubules with a ring-like diffraction patterns are common in MLC. The representative EDS spectrum of a single SnS−TaS2 tubule is shown in Figure S6 in the SI. SbS−TaS2 tubules. SbS−TaS2 tubules were synthesized with a yield close to 50%. Figure 8 shows big hedgehog-like agglomerates of SbS−TaS2 MLC tubular crystals (probably

with precise stoichiometry of (SbS)1.16TaS2 as reported for the bulk SbS−TaS2 MLC).7 The SbS and TaS2 layers are stacked periodically along the common c axis. At first sight, these agglomerates are reminiscent of Sn−S ones;17−19 however, many important differences exist. In contrast to the monosulfides of Sn (SnS) and Pb (PbS), no bulk phase of SbS is known. Rather, the stable antimony sulfide phase is Sb2S3 (stibnite) with Sb atoms in trivalent state. However, in misfit compounds, monolayers of SbS become stable. In analogy to SnS and PbS, SbS adopts a distorted NaCl 3765

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Figure 9. TEM images of (a) thin and (b) thicker SbS−TaS2 tubules with SbS and TaS2 layers stacked periodically along the c axis. Top: High and low (insets) magnification images. Middle: SAED patterns taken from the area shown in the high-magnification image. Tubule axis is marked by a pink double arrow. Spots pertinent to the same interplanar spacing are distributed on a ring-like patterns and their measured values are indicated. Blue arrows indicate the basal reflections produced from a superstructure. See text for small green arrows. Bottom: Line profile obtained from the region enclosed in the rectangle in the upper image.

triclinic distortion occurs with lattice parameters of a = 3.51 Å, b = 5.97 Å, c = 11.69 Å, α = 84.9°, β = 82.2°, γ = 89.7°7 and space group of C-1 (ICSD col. code 56496).5 The stabilization of the structure is attributed to a partial charge transfer from the SbS layer to TaS2 and interface modulation analogous to the misfit system (SbS)1.15TiS2.8

double layer structure, however, it exhibits triclinic distortion and is indexed in a triclinic system with lattice parameters of a = 5.9 Å, b = 6.15 Å, c = 11.6 Å, α = 84.5°, β = 84.5°, γ = 83.3° as detailed elsewhere.5,7,8 The space group is believed to be similar to SbS in (SbS)1.15TiS2 (ICSD col. code 56495), which is C-1.5 Also for TaS2, deviating from ortho-pseudohexagonal unit cell, 3766

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Figure 10. Representative X-ray diffraction pattern of the product obtained at the cold edge of the ampule after synthesis of SbS−TaS2 tubular crystals. The interplanar spacings and plane indices are marked on the spectrum. (a) Broad angle spectrum as a function of 2θ. (b) Portion of the spectrum as a function of the interplanar spacings.

ability to form these relatively large curvatures for inorganic compounds may shed some light on the structure and properties of these highly strained nanostructures. The lengths of the tubules varied between 1 μm and more then 20 μm. In most cases, telescopic growth steps were observed as clearly visible in Figure 8. It appears, therefore, that a variety of tubules with different sizes and morphologies (concentric ones, scrolllike, partially unfolded) are grown simultaneously in the same hedgehog-like agglomerate. The repeated appearance of the same tubular structures in different syntheses with different growth times, slightly different temperature gradients and precursor ratios supports this hypothesis. Additional nonstandard morphologies of tubular structures are shown in the SI in Figure S7. The formation of tubules is preceded by scrolls obtained from strained misfit SbS−TaS2 sheets as implied from Figure 8f as previously discussed for other misfit systems.18,19 However, other growth mechanisms for the tubules can not be excluded. It is also possible that a preformed thin tube with uniform outer diameter of 60−100 nm serves as a template for further scrolling of additional strained superstructure sheets. In a few cases, the outer diameter of the tubules shown in Figure 8 changes abruptly (Figure 8e middle and bottom); however, in most cases, wound envelopes are encountered as is clearly visible in Figure 8e (top) and b. Multistep nanotubes with varying outer and inner diameters are common for MLC as was also shown for PbS−NbS219 and SnS−SnS217−19 systems. Figure 9a shows a TEM image of a relatively thin SbS−TaS2 nanotube with outer and inner diameters of 65 and 25 nm, respectively. Figure 9b shows a thicker tube with 118 nm outer and 9 nm inner diameters. The c axis periodicity derived from the line profiles and ED patterns was found to be at the range of 11.7−12 Å (which is in fair agreement with the value of 11.54 Å with 0.3 Å FWHM obtained from XRD analysis; see below). The high intensity of the basal reflection in the ED patterns suggests highly ordered superstructure. Such reflections are also predominant in the XRD pattern. The spots with interplanar spacings of 1.66 and 2.87 Å are distributed on a circles in ED patterns and match the interplanar spacings of (11.0) and (10.0) planes of bulk 2H-TaS2 when indexed on pseudohexagonal coordinates (ICSD col. code 68488).41 The

The complex structure of the SbS was proposed on the basis of XRD measurements of the (SbS)1.15TiS2 crystals.5,7,8 One could distinguish two phases in the SbS subsystem. One phase is the normal one with rock-salt-like structure, which is found in other MLC as well. Here, each Sb atom is strongly bonded to five sulfur atoms in the SbS layer. The average Sb−S distance is 2.88 Å, which is similar to that in Sb2S3. Furthermore, the antimony atoms protrude from the sulfur planes on both sides of the SbS double layer. These structural characteristics leads to a partial coordination of the antimony atoms to the neighboring sulfur atoms in the TS2 (T = Ti, Nb, Ta) sublattice. Therefore, the antimony atoms are either 2- or 3-fold coordinated to the sulfur of the TS2 layer. The second phase is represented by additional modulation of the SbS region along [1−10]. This phase can be considered as an antiphase formed by zigzag chains of Sb−Sb and S−S bonds that build up at the antiphase boundaries. Calculations indicate that 22% of the Sb atoms are involved in the Sb−Sb clusters. The Sb−Sb distance is 2.84 Å and is similar to the Sb−Sb distance in the metal itself. Using XPS spectroscopy of bulk (SbS)1.15TiS2, it was shown that the main peaks correspond to the binding energies of the 3d3/2 and 3d5/2 states of the Sb atoms.8 They exhibit asymmetric widening toward lower energies relatively to their values in a pure Sb2S3 crystal. An analysis revealed two sets of peaks from the two Sb states (3d3/2 and 3d5/2) with a splitting energy of ∼1.1 eV for each. For the 3d levels, the difference between the trivalent Sb in Sb2S3 and the metallic Sb is about 1.2 eV. Therefore, from the peaks area calculation, it was suggested that 78% of the Sb atoms are trivalent as in Sb2S3, whereas the others (22%) are in a metallic-like state forming the so-called antiphase boundaries. Such a complex structure of the antimony chalcogenide was also believed to occur in a recently synthesized MLC (SbS1−xSex)1.16(Nb1.036S2)2 with (0.41 < x < 0.47).44 The outer diameters of the tubules range between 60 nm and up to ∼5 μm in several cases as shown in Figure 8. However, the typical outer diameters range between 100 and 800 nm. Interestingly, no strict correlation between the outer and the inner diameters was found and tubules with outer diameter of 200−300 nm and with inner one as small as ∼17 nm were often found. The smallest inner diameter found was ∼7 nm. The 3767

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yield was very small, and a very complex diffraction pattern was obtained that was not solved at present time, see SI Figure S11. The typically encountered outer diameters of PbS−TaS2 (150−250 nm), SbS−TaS2 (100−800 nm), and SnS−TaS2 (300−900 nm) tubes are larger than their PbS−NbS2 (50− 250)19 and SnS−SnS218 (20−60 nm) counterparts. Similar trend holds also for the maximum outer diameters. For equal thickness of the misfit slab, bigger diameters may arise from the higher “rigidity” of the misfit slab. The rigidity of the misfit slab is determined by a number of factors, like the separate Young’s modulus of the two (MX and TX2) components (the intralayer chemical bonds) and the nature of mutual interlayer interaction, which manifests in the following: (1) Mutual structural modulation that results in a different commensuration level between the MX and the TX2 layers. (2) The amount of possible charge transfer between the MX and the TX2 layers. (3) Possible covalent bonds between the MX and the TX2 layers. The general question regarding the stability of the misfit compounds and misfit nanotubes thereof is still an open question to some extent. Realization of nanotubular structures under variety of experimental conditions became evident only among several known misfit compounds, recently. It is highly believed that the misfit between the two components is the main driving force for the formation of tubular structures in addition to the healing of the dangling bonds at the (hk0) edges of the layers. With that notion in mind, the results presented here definitely point to a new research direction and reveal the great potential of this new field. The unique properties of such 1D nanostructures may lead for further research and attempts to synthesize tubular structures from other MLC.

spots with interplanar spacing of 2.15 and 1.96 Å are attributed to planes of SbS; however, no unambiguous indexes could be given at this time. Four sets of azimuthally splintered spots (marked by green arrows in SAED patterns) suggest 4-fold symmetry for these planes. The ring-like pattern of the reflections may suggest a scroll nature of the tubule with advancing difference between the lattice orientation at the bottom wall of the tube relatively to the upper one. On the other hand, these almost ring-like patterns were also obtained from tubules with constant diameters as shown in Figure 9 without any scrolling steps apparent. Such ring-like patterns were also obtained from partially unfolded SbS−TaS2 sheets (as shown in Figure S8 in the SI). Such observation suggests turbostratic misorientation of the SbS−TaS2 slabs about the common c axis precluding exact determination of the relative in-plane orientation between SbS and TaS2 layers. Upon stacking and mutual incommensurate modulation, SbS and TaS2 layers exhibit non matching triclinic unit cells5,7 with quite different axes lengths (lattice parameters) and angles. Therefore, the energy difference between multiple orientations between the SbS and TaS2 layers might be fairly low, and multiple possible orientations between the SbS and TaS2 layers within the different SbS−TaS2 slabs can be expected. However, because mutual structural modulation occurs in this MLC, it is reasonable to assume that such orientations are not fully random. Additional example of a thin, long SbS−TaS2 nanotube with uniform inner and outer diameters exhibiting ring-like diffraction pattern is shown in the SI in Figure S9. Chemical analysis of individual SbS−TaS2 nanotubes was carried out by energy dispersive spectroscopy (EDS) and representative spectrum is shown in Figure S10 in the SI. Figure 10a shows a representative X-ray diffraction pattern of the product obtained at the cold edge of the ampule following the attempted synthesis of SbS−TaS2 tubes. The interplanar spacings and plane indices are marked on the spectrum. The peak at 11.54 Å (0.3 Å FWHM) is pertinent to the (001) plane of the superstructure of SbS and TaS2 layers that are stacked periodically perpendicular to the common c axis. Higher orders (00n) are clearly visible and the reflections are consistent with the pattern obtained from planar (SbS)1.16TaS2 misfit microcrystals.34 The small peak at 6 Å matches the interplanar spacing of (00.1) planes of TaS2 (ICSD col. code 68488),41 which is indicative for the existence of small amounts of pure 2H−TaS2 phase as a byproduct. The peaks with interplanar spacings of 2.87 and 1.66 Å can arise from the existence of two different phases in the product. The first source is the “distorted” (10.0) and (11.0) planes of TaS2 layers that constitute the misfit compound tubular structures. The second source is the (10.0) and (11.0) planes of the residual plateletlike TaS2 phase. Because in the SbS−TaS2 misfit compound TaS2 suffers a triclinic distortion, TaS2 layers cannot be trivially indexed according to the pseudohexagonal system such as “10.0” and “11.0”. However, as was apparent from the ED patterns the interplanar spacings changes only slightly relative to bulk TaS2. Therefore, the “10.0” and “11.0” peaks at the XRD pattern are slightly broadened as shown by red arrows in Figure 10b. The peaks at 2.15 and 1.95 Å are attributed to SbS and are in a good agreement with the ED patterns. Attempts to synthesize BiS−TaS2 tubes were also undertaken using a similar synthetic approach. Indeed, nanotubes of this misfit compound were observed. However, their production

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CONCLUSIONS It was shown that PbS−TaS2, SnS−TaS2, SbS−TaS2, BiS−TaS2 MLC nano(micro)tubes could be obtained using a chemical vapor transport growth technique in evacuated quartz tubes under appropriate temperature gradient and reaction time. The formation of the concentric nanotubes initiates, most likely, with a formation of scrolls; however, additional mechanisms acting simultaneously can not be excluded. Selected area electron diffraction (SAED) shows that in the PbS−TaS2 (as well as in PbS−NbS2) system, frequently two types of TaS2 and three types of orientations of PbS layers exist in the same tubule. The two TaS2 layers are rotated relative to each other about the common c axis by 90° (30°). The b axes of the three PbS layers are parallel to the three equivalent b axes of one of the TaS2 layers and are rotated relative to each other by 60° (when indexed in ortho-pseudohexagonal system). Because the second TaS2 layer is rotated by 90° (30°) relative to the first, its b axis is parallel to the a axis of one of the PbS layers. Because in this case aPbS is almost equal to bPbS, the misfit between the aPbS and bTaS2 is rather small. This explains the two possible orientations between the PbS and TaS2 layers in the tubules of this misfit system. Due to the different orientation of the PbS and TaS2 layers about the common c axis, the b axes of several PbS and TaS2 layers will not coincide with the tubule axis. Therefore, the expectation from the tubule axis to coincide with one common b direction is not always valid. 3768

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(4) Wiegers, G. A.; Meerschaut, A. J. Alloys Compd. 1992, 178, 351. (5) Ren, Y.; Meetsma, A.; Petricek, V.; van Smaalen, S.; Wiegers, G. A. Acta Crystallogr.. 1995, B51, 275. (6) Wulff, J.; Meetsma, A.; Haange, R. J.; de Boer, J. L.; Wiegers, G. A. Synth. Met. 1990, 39, 1. (7) Espinos, J. P.; Gonzalez-Elipe, A. R.; Jumas, J. C.; OlivierFourcade, J.; Morales, J.; Tirado, J. L.; Lavela, P. Chem. Mater. 1997, 9, 1393. (8) Ren, Y.; Haas, C.; Wiegers, G. A. J. Phys.: Condens. Matter 1995, 7, 5949. (9) Wiegers, G. A.; Meetsma, A.; van Smaalen, S.; Haange, R. J.; Wulff, J.; Zeinstra, T.; de Boer, J. L.; Kuypers, S.; Van Tendeloo, G.; Van Landuyt, J.; Amelinckx, S.; Meerschaut, A.; Rabu, P.; Rouxel, J. Solid State Commun. 1989, 70, 409. (10) Wiegers, G. A.; Meetsma, A.; Haange, R. J.; van Smaalen, S.; de Boer, J. L. Acta Crystallogr. 1990, B46, 324. (11) Jobst, A.; Van Smaalen, S. Acta Crystallogr. 2002, B58, 179. (12) Wiegers, G. A. Phys. B (Amsterdam, Neth.) 1980, 99, 151. (13) Kato, K. Acta Crystallogr. 1990, B46, 39. (14) Gotoh, Y.; Goto, M.; Kawaguchi, K.; Oosawa, Y.; Onoda, M. Mater. Res. Bull. 1990, 25, 307. (15) Onoda, M.; Kato, K.; Gotoh, Y.; Oosawa, Y. Acta Crystallogr. 1990, B46, 487. (16) de Boer, J. L.; Meetsma, A.; Zeinstra, T. J.; Haange, R. J.; Wiegers, G. A. Acta Crystallogr. 1991, C47, 924. (17) Radovsky, G.; Popovitz-Biro, R.; Staiger, M.; Gartsman, K.; Thomsen, C.; Lorenz, T.; Seifert, G.; Tenne, R. Angew. Chem., Int. Ed. 2011, 50, 12316. (18) Radovsky, G.; Popovitz-Biro, R.; Tenne, R. Chem. Mater. 2012, 24, 3004. (19) Radovsky, G.; Popovitz-Biro, R.; Stroppa, D. G.; Houben, L.; Tenne, R. Acc. Chem. Res. 2014, 47, 406. (20) Wiegers, G. A. J. Alloys Compd. 1995, 219, 152. (21) Hangyo, M.; Nakashima, S.; Hamada, Y.; Nishio, T. Phys. Rev. B 1993, 48, 11291. (22) Kallane, M.; Rossnagel, K.; Marczynski-Buhlow, M.; Kipp, L. Phys. Rev. Lett. 2008, 100, 065502. (23) Ohno, Y. Phys. Rev. B 1991, 44, 1281. (24) Ettema, A. R. H. F.; Haas, C. J. Phys.: Condens. Matter 1993, 5, 3817. (25) Moelo, Y.; Meerschaut, A.; Rouxel, J.; Auriel, C. Chem. Mater. 1995, 7, 1759. (26) Meerschaut, A.; Moelo, Y.; Cario, L.; Lafond, A.; Deudon, C. Mol. Cryst. Liq. Cryst. Bull. 2000, 341, 1. (27) Brandt, J.; Kipp, L.; Skibowski, M.; Krasovskii, E. E.; Schattke, W.; Spiecker, E.; Dieker, C.; Jäger, W. Surf. Sci. 2003, 705, 532. (28) Wiegers, G. A. Jpn. J. Appl. Phys. 1993, 32, 705. (29) Hangyo, M.; Kisoda, K.; Nakashima, S.; Meerschaut, A.; Rouxel, J. Phys. B 1996, 219−220, 481. (30) Fang, C. M.; Ettema, A. R. H. F.; Haas, C.; Wiegers, G. A. Phys. Rev. B 1995, 52, 2336. (31) Kabliman, E.; Blaha, P.; Schwarz, K. Phys. Rev. B 2010, 82, 125308. (32) Reefman, D.; Baak, J.; Brom, H. B.; Wiegers, G. A. Solid State Commun. 1990, 75, 47. (33) Meerschaut, A.; Rabu, P.; Rouxel, J. Mater. Res. Bull. 1990, 25, 855. (34) Gotoh, Y.; Onoda, M.; Akimoto, J.; Oosawa, Y. Jpn. J. Appl. Phys. 1991, 30, L1039. (35) Katayama, N.; Nohara, M.; Tagaki, H. Phys. C 2006, 35−38, 445. (36) Kociak, M.; Kasumov, A. Y.; Guéron, S.; Reulet, B.; Khodos, I. I.; Gorbatov, Y. B.; Volkov, V. T.; Vaccarini, L.; Bouchiat, H. Phys. Rev. Lett. 2001, 86, 2416. (37) Bernaerts, D.; Amelinckx, S.; van Tendeloo, G.; van Landuyt, J. J. Cryst. Growth 1997, 172, 433. (38) Jellinek, F. J. Less Common Met. 1962, 4, 9. (39) Revelli, J. F. Inorg. Synth. 1979, 19, 35.

The internal structure of the SnS−TaS2 tubes have much in common with their PbS−TaS2 counterparts. This fact is attributed to isostructural interface modulation of the SnS layers as their PbS counterparts within stacking with TaS2. In the SbS−TaS2 system, the relative in-plane orientation could not be determined. The ring-like patterns can arise from the either multiple orientations between the SbS and the TaS2 layers or either from turbostratic misorientation of the SbS− TaS2 slabs. The multiple orientations within the slabs are favored by a high degree of the misfit between two triclinic cells of the SbS and TaS2. However, even if multiple orientations between the SbS and TaS2 layers within the slabs occur, they are not believed to be random. It was shown that tubular nanocrystals synthesized from misfit compounds of different materials have much in common. (1) The presence of partly unfolded sheets. (2) The presence of a variety of internal structures common to all tubules of different compositions. (3) The presence of dislocation-like defects. More rarely encountered in PbS−TaS2, SbS−TaS2, SnS−TaS2 and more commonly in SnS−SnS2 tubes (not shown here), such dislocations are indicative of transition from the scroll-like to concentric tubules.

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ASSOCIATED CONTENT

* Supporting Information S

PbS−TaS2 scroll with dislocation-like defects; PbS−TaS2 tubules with one and two types of folding vectors of PbS layers; chemical analysis of a single PbS−TaS2 nanotubes; partially unrolled SnS−TaS2 scroll-like tubule with a ring-like diffraction pattern; chemical analysis of a single SnS−TaS2 nanotubes; additional SbS−TaS2 thick tubules with “special” morphologies; partially folded SbS−TaS2 sheet with ring-like diffraction pattern; additional SbS−TaS2 thin tubule; chemical analysis of a single SbS−TaS2 nanotubes; BiS−TaS2 tubules. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +972-8-9342394. Fax: +972-8-934-4138. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Yishay Feldman for XRD measurements. This research was supported by the ERC grant INTIF 226639; the EU ITN 317451 grant and a grant of the Israel Science Foundation. R.T. acknowledges the support of the Harold Perlman and the Irving and Azelle Waltcher Foundations and the Irving and Cherna Moskowitz Center for Nano and BioNano Imaging. He holds the Drake Family chair in Nanotechnology and is the director of the Helen and Martin Kimmel Center for Nanoscale Science.

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REFERENCES

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