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High-temperature fibres composed of transition metal inorganic nanotubes
Current Opinion in Solid State and Materials Science 8 (2004) 121–125
High-temperature fibres composed of transition metal inorganic nanotubes Maja Remskar *, Ales Mrzel J. Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 30 March 2003; accepted 30 January 2004
Abstract Chemical transport reaction enables growth of pure and alloyed inorganic nanotubes with extremely low structural defect densities and exceptional length. The narrow nanotubes self-assemble into ropes by syntactic coalescence. The mechanical and thermal stability of WS2 nanotubes are discussed and some possible applications are suggested. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Nanotubes; MoS2 ; WS2 ; Self-assembly; Nanowires; Strengthening fibres
1. Introduction Inorganic nanotubes represent a quickly developing field of nanotube investigations. In 1992, the first successful growth of WS2 nanotubes was reported by Tenne’s group [1]. The report issued only one year after the discovery of carbon nanotubes. Unfortunately, since the intensive research of carbon nanotubes in the last decade has attracted mass attention and financial support, inorganic nanotubes tended to be under researched in the early years after their discovery. In 1996, inorganic nanotubes were first synthesised by chemical transport reaction, which is a standard method for growth of transition metal dichalcogenides (TMD) [2]. More recently, the number of reports on synthesis of different inorganic nanotubes has increased weekly, revealing the importance of this field for nanotechnology. The important knowledge accumulated in the scientific groups during the research of carbon nanotubes has facilitated the understanding of inorganic nanotubes and enabled fast development in their synthesis and possible applications. Although they share some geometrical similarities with carbon nanotubes, TMD nanotubes represent a unique system with important features,
* Corresponding author. Tel.: +386-1-4773-728; fax: +386-1-2519385. E-mail addresses: [email protected] (M. Remskar), ales.mrzel@ ijs.si (A. Mrzel).
1359-0286/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2004.01.004
from the growth mechanisms to the physical and chemical properties. Numerous growth techniques are used for the synthesis of transition metal dichalcogenide inorganic nanotubes (NTs). After initial enthusiasm following the successful synthesis of cylindrical crystals from this new compound, additional demands became apparent with respect to their properties. The control of their dimensions is desired and crucial for some applications. The second important demand is their structural perfection, especially in order to achieve the mechanical and electric properties significant in construction of nanodevices. Both these demands are rarely satisfied simultaneously in inorganic and carbon nanotubes. The longitudinal length of NTs, which were synthesised by reactor techniques using the sulphurisation of oxides, was determined by the geometry of the precursor crystals [3]. The dimensions of the precursor crystals also dictate the size of NTs during the reduction of needle-like transition metal trisulphides and recrystallization to the disulphide cylindrical structure [4]. They can be up to several tens of micrometers in length with diameters from a few tens of nanometers up to hundreds of nanometers. In addition, the dimensions of TMD nanotubes can be controlled using a template method with thermal decomposition of (NH4 )2 MoS4 and (NH4 )2 Mo3 S13 in porous aluminium oxide membrane [5]. The MoS2 NTs produced by in this way were all of approximately the same size: 30 nm long with diameter of 50 nm and wall thickness of about 10 nm.
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M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
The other growth techniques available lead to the growth of nanotubes with a wide range of diameters and lengths. By far the longest TMD nanotubes, with extremely low density of structural defects, were produced by chemical transport reaction. The growth of MoS2 [2] and WS2 [6] NTs takes place at a very slow rate from the vapour phase. The tubes grow up to several millimeters in length. The diameters in multiwall NTs range from several micrometers to less than ten nanometers. Whereas microtubes usually grow as single tubes, the nanotubes frequently combine and build ropes. Nanoropes form by co-axial or side by side growth of nanotubes, by selfassembly of single tubes and by coalescence of single tubes at low angles of incidence. The evidence of syntactic growth of nanotubes in the form of ropes was reported, where the crystal lattice of a particular tube defines the geometry and orientation of other tubes in the rope [7].
2. Experimental Chemical transport reaction is a standard method for growing plate-like crystals of TMD layered crystals in a two zone furnace. Modifications of growth parameters, for example temperature, temperature gradient, and/or partial pressures of components in the transport reaction change the crystal growth from a plane geometry to a cylindrical one. The instability of weakly bonded crystalline sheets which are prone to folding, and the saturation of dangling bonds in the self-terminated planes, tend to stabilise cylindrical crystal shapes rather than flat geometries. The MoS2 and WS2 NTs grow together with plate-like crystals at 103 Pa with a temperature gradient of 5.6 K/ cm [2,6]. The transport agent, iodine in our case, reacts with the transition metal at high temperature (above 1120 K) forming the volatile product, which decomposes at a lower temperature (1010 K), where the transition metal reacts again with sulphur to form the solid transition metal disulphide. In three weeks, a few percent of the starting material was transported by the reaction to form nanotubes, while the rest of the transported material formed strongly undulated thin plate-like crystals.
3. Geometry and lattice structure 3.1. Crystal structure of transition metal dichalcogenide layered compounds MoS2 and WS2 compounds belong to the Group VI family of transition metal dichalcogenides [8]. They are extremely non-isotropic solids with a layer type structure. The weak interaction holding the layers is predominantly of the van der Waals type. The molecular layer S–Mo–S or S–W–S shows a trigonal symmetry. The transition
Fig. 1. Two possible stacking schemes of adjacent MoS2 and WS2 molecular layers: (a) three-dimensional model of the stable 2Hb stacking; (b) and (c) the [1 1 2 0] sections of 2H and 3R stacking, respectively.
metal atom is co-ordinated by six sulphur atoms situated at the corners of a trigonal prism (Fig. 1). The molecular layers can be stacked in two ways, as a hexagonal polytype 2Hb with two molecular layers (P63 /mmc) and as rhombohedral polytype 3R with three molecular layers per unit cell (R3m). Under special conditions both polytypes can nucleate simultaneously [9]. The origin of the undulation of the very thin crystal flakes, which constitute the nucleation sites for the tubes, is explained by a stacking mismatch between nucleation islands of the rhombohedral and hexagonal stacking, which causes internal strain [7]. Thin folded flakes can roll up and adopt the cylindrical shape of tubes directly. Alternatively, they can also produce a turbulent gas flow of transported molecules, which also promotes a tube-like growth mode. When the tube form is nucleated, the tube continues to grow up to relatively large distances. The growth is rigid, resulting in straight tubes of homogeneous diameter. 3.2. Lattice structure of MoS2 and WS2 multiwalled nanotubes Non-alloyed MoS2 and WS2 nanotubes are always found in the helical forms [2,6,7]. Projection of a tube axis onto the basal [0 0 0 1] plane of the wall is rotated with respect to the h1 0 1 0i lattice directions for an angle of chirality a. Different chiralities were found in the nanotubes, from a few degrees up to 20°. Frequently, several chiralities are present even within the same nanotube, especially in the thick-walled nanotubes with diameters below 100 nm. The contraction of interlayer distances caused by internal strain incorporated in the tube wall influences the chirality of the nanotubes. Each winding of the
M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
123
Fig. 2. A WS2 nanotube, 76 nm in diameter, terminated by a steep crater with a small truncated cone (insert) in the central part of the tube.
molecular layer of thickness t, i.e. 0.6147 nm for MoS2 [8], enlarges the tube circumference by 2pt. Since this value is not an integer multiple of the lattice parameter a in the basal plane (0.316 nm), the molecular layers have to be strained and/or helical, assuming that a regular crystal lattice is formed at least in narrow strips parallel with the nanotube axis. Increasing strain intensity toward the nanotube central hole causes the co-existence of several chiralities in the same nanotube. In thick-wall nanotubes the incorporated strain causes the reduction of the interlayer distances, in some tubes even below the value typical for plate-like crystals. In such cases the longitudinal growth of the nanotubes is capped with the typical crater-cone terminations, for example, a thick-walled nanotube, 76 nm in diameter, is terminated with the geometry of a steep crater with a small truncated cone in the central part of the tube (Fig. 2). The diameter of the inner hole is less than 2 nm. At the top of the truncated cone the hole is widened because the intensity of the incorporated strain is reduced due to the decreasing number of rolled up molecular layers (insert). The abrupt change of chirality affecting the strain relaxation can change the geometry of nanotubes and can stop the longitudinal growth. The thickness of a nanotube wall dictates its chirality and vice versa, i.e. the chirality influences the wall thickness.
4. Mechanical stability of MoS2 and WS2 nanotubes and nanoribbons against tensile forces While TMD nanotubes produced by chemical transport reaction are soft with respect to radial forces, they show extremely large stability against tensile forces. Quantitative data are still not available, but qualitative
Fig. 3. An artificial association of WS2 nanotubes reveals the strong stability against tensile forces.
estimation ranks these materials amongst candidates for high-temperature strengthening fibres. The association of WS2 nanotubes shown in Fig. 3 was made by axial rotation of tweezers holding the strand of nanotubes. The nanotubes, even the smallest in the strand with diameters below 30 nm did not break apart during this robust processhandling. Their mechanical stability is explained by their chiral growth mode and high lattice perfection, which mean that the transport of dislocations in a radial direction needed in order to break the material is rendered more difficult. A rough estimation of the Young’s modulus for a nanotube, 30 nm in diameter and 10 nm in wall thickness with estimated force 0.01 N, gives a value two orders of magnitude larger than that measured in sputtered MoS2 layered crystallites (160 GPa) [10]. The available data [10] originate from nanoindentation studies with strong contributions of elastic constants perpendicular to the MoS2 layers. In contrast, the longitudinal tensile forces applied to the nanotubes in our experiment influence the stretching in the basal plane, resulting in larger values of Young’s modulus.
5. Thermal stability of nanotubes under electron beam irradiation The MoS2 and WS2 nanotubes grow at a relatively high temperature (1010 K) in an iodine and sulphur rich local atmosphere. Annealing of the nanotubes in air results in complete oxidation at 570 K. The cylindrical frame remains, but the wall fragmentizes to MoO3 or WO3 nanocrystallites. In situ annealing in a high vacuum inside a transmission electron microscope (103 Pa)
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M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
by irradiation with an electron beam enables the observation of progressive decomposition of the nanotubes. Considering that the quality of an electrical junction of a nanotube with the copper grid is not known, the local temperatures can be only estimated. The WS2 nanotube, 35 nm in diameter, 5 nm in wall thickness and composed of 8 molecular layers, was completely stable during irradiation with a dose 7 A/cm2 (Fig. 4a). A dose increased by around 6 times (50 A/cm2 ) and applied for two minutes started the decomposition process of the molecular layers (Fig. 4b). The destruction of the nanotube takes place layer by layer from the outside toward the inner layers. Some outer molecular layers are completely removed, what indicates indirectly that the temperature above 1120 K, where the WO3 becomes volatile (figure inset). An increased electron dose up to 180 A/cm2 for additional two minutes destroyed cylindrical geometry; the diameter shrunk by nearly 50% and the tube became partially oxidized. The corresponding electron diffraction pattern (Fig. 5) is a superposition of electrons scattered by both walls of the tube oriented perpendicular to the electron beam and by side parts of the wall oriented parallel with the beam. The tube wall was composed of chirally grown molecular layers with a 13° chiral angle and non-chiral molecular layers (Fig. 5a). After the irradiation with the maximum dose the diffraction spots belonging to scattering by the side (0 0 l) MoS2 planes are much weaker than before the irradiation, revealing the destruction of the cylindrical geometry. In addition, the position of the {1 0 0} spots are turned by 10° indicating the twisting of the nanotube during the annealing. The screw deformation is explained by relaxation of internal strain incorporated in the chirally grown nanotube wall.
Fig. 5. The transmission electron diffraction pattern of electrons scattered by a WS2 nanotube: (a) at low dose of electrons––7 A/cm2 ; (b) at 180 A/cm2 .
The relatively high thermal stability of inorganic nanotubes in a non-oxidizing atmosphere is concluded from the observation that only 50 nm away from the irradiation spot the nanotube was almost completely unaffected (Fig. 4c––upper part of the image). The origin of the decomposition was obviously not only the enhanced temperature, but also the irradiation damage of the lattice by the 200 keV electrons, which trigger the local decomposition of the wall. 6. Conclusions In comparison with other growth techniques, the advantage of the lasting chemical transport reaction lies in the fact that the nanotubes grown by this technique contain an extremely low density of structural defects. The tubes grow up to several millimeter lengths with diameters ranging from several micrometers to less than ten nanometers. They grow alongside plate-like crystals
Fig. 4. Irradiation of the WS2 nanotube, 35 nm in diameter, by an electron beam in a transmission electron microscope. Density of electrons and duration of irradiation: (a) 7 A/cm2 ––stable structure; (b) 50 A/cm2 ––2 min; (c) 180 A/cm2 ––2 min.
M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
of the same compound. The diameter, wall thickness and length are determined by the local microenvironment during the growth process. Due to their cylindrical hollow geometry, inorganic nanotubes have a low mass density, a high porosity and an extremely large surface to weight ratio. Their potential applications range from high porous catalytic and ultralight anticorrosive materials, atomic probes and field emitters, to high-temperature strengthening fibres. The helical structure of undoped tubes with semiconductor behavior and their optical activity enable possible applications in non-linear optics and in solar cell technology. By functionalizing the nanotubes, e.g. using specific Mo containing enzymes, novel functional biomaterials could be made.
References [1] Tenne R, Margulis L, Genut M, Hodes G. Polyhedral and cylindrical structures of tungsten disulfide. Nature 1992;360:444–6.
125
[2] Remskar M, Skraba Z, Cleton F, Sanjines R, Levy F. MoS2 as microtubes. Appl Phys Lett 1996;69:351–3. [3] Rothschild A, Popovitz-Biro R, Lourie O, Tenne R. Morphology of multiwall WS2 nanotubes. J Phys Chem B 2000;104:8976– 81. [4] Nath M, Govindaray A, Rao CNR. Simple synthesis of MoS2 and WS2 nanotubes. Adv Mater 2001;13:283. [5] Zelenski CM, Dorhout PK. Template synthesis of near-monodisperse microscale nanofibers and nanotubules of MoS2 . J Am Chem Soc 1998;120:734–42. [6] Remskar M, Skraba Z, Ballif C, Sanjines R, Levy F. New crystal structures of WS2 : microtubes, ribbons, and ropes. Adv Mater 1998;10:246–9. [7] Remskar M, Skraba Z, Sanjines R, Levy F. Syntactic coalescence of WS2 nanotubes. Appl Phys Lett 1999;74:3633–5. [8] Wilson JA, Yoffe AD. The transition metal dichalcogenides. Adv Phys 1969;18:193–335. [9] Remskar M, Skraba Z, Ballif C, Sanjines R, Levy F. Stabilization of the rhombohedral polytype in MoS2 and WS2 microtubes: TEM and AFM study. Surf Sci 1999;433–435:637– 41. [10] Le Mogne T, Donnet C, Martin JM, Tonck A, Millard-Pinard N, Fayeulle S, et al. Nature of super-lubricating MoS2 physical vapor deposition coatings. J Vac Sci Tech A 1994;12:1998– 2004.
High-temperature fibres composed of transition metal inorganic nanotubes Maja Remskar *, Ales Mrzel J. Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 30 March 2003; accepted 30 January 2004
Abstract Chemical transport reaction enables growth of pure and alloyed inorganic nanotubes with extremely low structural defect densities and exceptional length. The narrow nanotubes self-assemble into ropes by syntactic coalescence. The mechanical and thermal stability of WS2 nanotubes are discussed and some possible applications are suggested. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Nanotubes; MoS2 ; WS2 ; Self-assembly; Nanowires; Strengthening fibres
1. Introduction Inorganic nanotubes represent a quickly developing field of nanotube investigations. In 1992, the first successful growth of WS2 nanotubes was reported by Tenne’s group [1]. The report issued only one year after the discovery of carbon nanotubes. Unfortunately, since the intensive research of carbon nanotubes in the last decade has attracted mass attention and financial support, inorganic nanotubes tended to be under researched in the early years after their discovery. In 1996, inorganic nanotubes were first synthesised by chemical transport reaction, which is a standard method for growth of transition metal dichalcogenides (TMD) [2]. More recently, the number of reports on synthesis of different inorganic nanotubes has increased weekly, revealing the importance of this field for nanotechnology. The important knowledge accumulated in the scientific groups during the research of carbon nanotubes has facilitated the understanding of inorganic nanotubes and enabled fast development in their synthesis and possible applications. Although they share some geometrical similarities with carbon nanotubes, TMD nanotubes represent a unique system with important features,
* Corresponding author. Tel.: +386-1-4773-728; fax: +386-1-2519385. E-mail addresses: [email protected] (M. Remskar), ales.mrzel@ ijs.si (A. Mrzel).
1359-0286/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2004.01.004
from the growth mechanisms to the physical and chemical properties. Numerous growth techniques are used for the synthesis of transition metal dichalcogenide inorganic nanotubes (NTs). After initial enthusiasm following the successful synthesis of cylindrical crystals from this new compound, additional demands became apparent with respect to their properties. The control of their dimensions is desired and crucial for some applications. The second important demand is their structural perfection, especially in order to achieve the mechanical and electric properties significant in construction of nanodevices. Both these demands are rarely satisfied simultaneously in inorganic and carbon nanotubes. The longitudinal length of NTs, which were synthesised by reactor techniques using the sulphurisation of oxides, was determined by the geometry of the precursor crystals [3]. The dimensions of the precursor crystals also dictate the size of NTs during the reduction of needle-like transition metal trisulphides and recrystallization to the disulphide cylindrical structure [4]. They can be up to several tens of micrometers in length with diameters from a few tens of nanometers up to hundreds of nanometers. In addition, the dimensions of TMD nanotubes can be controlled using a template method with thermal decomposition of (NH4 )2 MoS4 and (NH4 )2 Mo3 S13 in porous aluminium oxide membrane [5]. The MoS2 NTs produced by in this way were all of approximately the same size: 30 nm long with diameter of 50 nm and wall thickness of about 10 nm.
122
M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
The other growth techniques available lead to the growth of nanotubes with a wide range of diameters and lengths. By far the longest TMD nanotubes, with extremely low density of structural defects, were produced by chemical transport reaction. The growth of MoS2 [2] and WS2 [6] NTs takes place at a very slow rate from the vapour phase. The tubes grow up to several millimeters in length. The diameters in multiwall NTs range from several micrometers to less than ten nanometers. Whereas microtubes usually grow as single tubes, the nanotubes frequently combine and build ropes. Nanoropes form by co-axial or side by side growth of nanotubes, by selfassembly of single tubes and by coalescence of single tubes at low angles of incidence. The evidence of syntactic growth of nanotubes in the form of ropes was reported, where the crystal lattice of a particular tube defines the geometry and orientation of other tubes in the rope [7].
2. Experimental Chemical transport reaction is a standard method for growing plate-like crystals of TMD layered crystals in a two zone furnace. Modifications of growth parameters, for example temperature, temperature gradient, and/or partial pressures of components in the transport reaction change the crystal growth from a plane geometry to a cylindrical one. The instability of weakly bonded crystalline sheets which are prone to folding, and the saturation of dangling bonds in the self-terminated planes, tend to stabilise cylindrical crystal shapes rather than flat geometries. The MoS2 and WS2 NTs grow together with plate-like crystals at 103 Pa with a temperature gradient of 5.6 K/ cm [2,6]. The transport agent, iodine in our case, reacts with the transition metal at high temperature (above 1120 K) forming the volatile product, which decomposes at a lower temperature (1010 K), where the transition metal reacts again with sulphur to form the solid transition metal disulphide. In three weeks, a few percent of the starting material was transported by the reaction to form nanotubes, while the rest of the transported material formed strongly undulated thin plate-like crystals.
3. Geometry and lattice structure 3.1. Crystal structure of transition metal dichalcogenide layered compounds MoS2 and WS2 compounds belong to the Group VI family of transition metal dichalcogenides [8]. They are extremely non-isotropic solids with a layer type structure. The weak interaction holding the layers is predominantly of the van der Waals type. The molecular layer S–Mo–S or S–W–S shows a trigonal symmetry. The transition
Fig. 1. Two possible stacking schemes of adjacent MoS2 and WS2 molecular layers: (a) three-dimensional model of the stable 2Hb stacking; (b) and (c) the [1 1 2 0] sections of 2H and 3R stacking, respectively.
metal atom is co-ordinated by six sulphur atoms situated at the corners of a trigonal prism (Fig. 1). The molecular layers can be stacked in two ways, as a hexagonal polytype 2Hb with two molecular layers (P63 /mmc) and as rhombohedral polytype 3R with three molecular layers per unit cell (R3m). Under special conditions both polytypes can nucleate simultaneously [9]. The origin of the undulation of the very thin crystal flakes, which constitute the nucleation sites for the tubes, is explained by a stacking mismatch between nucleation islands of the rhombohedral and hexagonal stacking, which causes internal strain [7]. Thin folded flakes can roll up and adopt the cylindrical shape of tubes directly. Alternatively, they can also produce a turbulent gas flow of transported molecules, which also promotes a tube-like growth mode. When the tube form is nucleated, the tube continues to grow up to relatively large distances. The growth is rigid, resulting in straight tubes of homogeneous diameter. 3.2. Lattice structure of MoS2 and WS2 multiwalled nanotubes Non-alloyed MoS2 and WS2 nanotubes are always found in the helical forms [2,6,7]. Projection of a tube axis onto the basal [0 0 0 1] plane of the wall is rotated with respect to the h1 0 1 0i lattice directions for an angle of chirality a. Different chiralities were found in the nanotubes, from a few degrees up to 20°. Frequently, several chiralities are present even within the same nanotube, especially in the thick-walled nanotubes with diameters below 100 nm. The contraction of interlayer distances caused by internal strain incorporated in the tube wall influences the chirality of the nanotubes. Each winding of the
M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
123
Fig. 2. A WS2 nanotube, 76 nm in diameter, terminated by a steep crater with a small truncated cone (insert) in the central part of the tube.
molecular layer of thickness t, i.e. 0.6147 nm for MoS2 [8], enlarges the tube circumference by 2pt. Since this value is not an integer multiple of the lattice parameter a in the basal plane (0.316 nm), the molecular layers have to be strained and/or helical, assuming that a regular crystal lattice is formed at least in narrow strips parallel with the nanotube axis. Increasing strain intensity toward the nanotube central hole causes the co-existence of several chiralities in the same nanotube. In thick-wall nanotubes the incorporated strain causes the reduction of the interlayer distances, in some tubes even below the value typical for plate-like crystals. In such cases the longitudinal growth of the nanotubes is capped with the typical crater-cone terminations, for example, a thick-walled nanotube, 76 nm in diameter, is terminated with the geometry of a steep crater with a small truncated cone in the central part of the tube (Fig. 2). The diameter of the inner hole is less than 2 nm. At the top of the truncated cone the hole is widened because the intensity of the incorporated strain is reduced due to the decreasing number of rolled up molecular layers (insert). The abrupt change of chirality affecting the strain relaxation can change the geometry of nanotubes and can stop the longitudinal growth. The thickness of a nanotube wall dictates its chirality and vice versa, i.e. the chirality influences the wall thickness.
4. Mechanical stability of MoS2 and WS2 nanotubes and nanoribbons against tensile forces While TMD nanotubes produced by chemical transport reaction are soft with respect to radial forces, they show extremely large stability against tensile forces. Quantitative data are still not available, but qualitative
Fig. 3. An artificial association of WS2 nanotubes reveals the strong stability against tensile forces.
estimation ranks these materials amongst candidates for high-temperature strengthening fibres. The association of WS2 nanotubes shown in Fig. 3 was made by axial rotation of tweezers holding the strand of nanotubes. The nanotubes, even the smallest in the strand with diameters below 30 nm did not break apart during this robust processhandling. Their mechanical stability is explained by their chiral growth mode and high lattice perfection, which mean that the transport of dislocations in a radial direction needed in order to break the material is rendered more difficult. A rough estimation of the Young’s modulus for a nanotube, 30 nm in diameter and 10 nm in wall thickness with estimated force 0.01 N, gives a value two orders of magnitude larger than that measured in sputtered MoS2 layered crystallites (160 GPa) [10]. The available data [10] originate from nanoindentation studies with strong contributions of elastic constants perpendicular to the MoS2 layers. In contrast, the longitudinal tensile forces applied to the nanotubes in our experiment influence the stretching in the basal plane, resulting in larger values of Young’s modulus.
5. Thermal stability of nanotubes under electron beam irradiation The MoS2 and WS2 nanotubes grow at a relatively high temperature (1010 K) in an iodine and sulphur rich local atmosphere. Annealing of the nanotubes in air results in complete oxidation at 570 K. The cylindrical frame remains, but the wall fragmentizes to MoO3 or WO3 nanocrystallites. In situ annealing in a high vacuum inside a transmission electron microscope (103 Pa)
124
M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
by irradiation with an electron beam enables the observation of progressive decomposition of the nanotubes. Considering that the quality of an electrical junction of a nanotube with the copper grid is not known, the local temperatures can be only estimated. The WS2 nanotube, 35 nm in diameter, 5 nm in wall thickness and composed of 8 molecular layers, was completely stable during irradiation with a dose 7 A/cm2 (Fig. 4a). A dose increased by around 6 times (50 A/cm2 ) and applied for two minutes started the decomposition process of the molecular layers (Fig. 4b). The destruction of the nanotube takes place layer by layer from the outside toward the inner layers. Some outer molecular layers are completely removed, what indicates indirectly that the temperature above 1120 K, where the WO3 becomes volatile (figure inset). An increased electron dose up to 180 A/cm2 for additional two minutes destroyed cylindrical geometry; the diameter shrunk by nearly 50% and the tube became partially oxidized. The corresponding electron diffraction pattern (Fig. 5) is a superposition of electrons scattered by both walls of the tube oriented perpendicular to the electron beam and by side parts of the wall oriented parallel with the beam. The tube wall was composed of chirally grown molecular layers with a 13° chiral angle and non-chiral molecular layers (Fig. 5a). After the irradiation with the maximum dose the diffraction spots belonging to scattering by the side (0 0 l) MoS2 planes are much weaker than before the irradiation, revealing the destruction of the cylindrical geometry. In addition, the position of the {1 0 0} spots are turned by 10° indicating the twisting of the nanotube during the annealing. The screw deformation is explained by relaxation of internal strain incorporated in the chirally grown nanotube wall.
Fig. 5. The transmission electron diffraction pattern of electrons scattered by a WS2 nanotube: (a) at low dose of electrons––7 A/cm2 ; (b) at 180 A/cm2 .
The relatively high thermal stability of inorganic nanotubes in a non-oxidizing atmosphere is concluded from the observation that only 50 nm away from the irradiation spot the nanotube was almost completely unaffected (Fig. 4c––upper part of the image). The origin of the decomposition was obviously not only the enhanced temperature, but also the irradiation damage of the lattice by the 200 keV electrons, which trigger the local decomposition of the wall. 6. Conclusions In comparison with other growth techniques, the advantage of the lasting chemical transport reaction lies in the fact that the nanotubes grown by this technique contain an extremely low density of structural defects. The tubes grow up to several millimeter lengths with diameters ranging from several micrometers to less than ten nanometers. They grow alongside plate-like crystals
Fig. 4. Irradiation of the WS2 nanotube, 35 nm in diameter, by an electron beam in a transmission electron microscope. Density of electrons and duration of irradiation: (a) 7 A/cm2 ––stable structure; (b) 50 A/cm2 ––2 min; (c) 180 A/cm2 ––2 min.
M. Remskar, A. Mrzel / Current Opinion in Solid State and Materials Science 8 (2004) 121–125
of the same compound. The diameter, wall thickness and length are determined by the local microenvironment during the growth process. Due to their cylindrical hollow geometry, inorganic nanotubes have a low mass density, a high porosity and an extremely large surface to weight ratio. Their potential applications range from high porous catalytic and ultralight anticorrosive materials, atomic probes and field emitters, to high-temperature strengthening fibres. The helical structure of undoped tubes with semiconductor behavior and their optical activity enable possible applications in non-linear optics and in solar cell technology. By functionalizing the nanotubes, e.g. using specific Mo containing enzymes, novel functional biomaterials could be made.
References [1] Tenne R, Margulis L, Genut M, Hodes G. Polyhedral and cylindrical structures of tungsten disulfide. Nature 1992;360:444–6.
125
[2] Remskar M, Skraba Z, Cleton F, Sanjines R, Levy F. MoS2 as microtubes. Appl Phys Lett 1996;69:351–3. [3] Rothschild A, Popovitz-Biro R, Lourie O, Tenne R. Morphology of multiwall WS2 nanotubes. J Phys Chem B 2000;104:8976– 81. [4] Nath M, Govindaray A, Rao CNR. Simple synthesis of MoS2 and WS2 nanotubes. Adv Mater 2001;13:283. [5] Zelenski CM, Dorhout PK. Template synthesis of near-monodisperse microscale nanofibers and nanotubules of MoS2 . J Am Chem Soc 1998;120:734–42. [6] Remskar M, Skraba Z, Ballif C, Sanjines R, Levy F. New crystal structures of WS2 : microtubes, ribbons, and ropes. Adv Mater 1998;10:246–9. [7] Remskar M, Skraba Z, Sanjines R, Levy F. Syntactic coalescence of WS2 nanotubes. Appl Phys Lett 1999;74:3633–5. [8] Wilson JA, Yoffe AD. The transition metal dichalcogenides. Adv Phys 1969;18:193–335. [9] Remskar M, Skraba Z, Ballif C, Sanjines R, Levy F. Stabilization of the rhombohedral polytype in MoS2 and WS2 microtubes: TEM and AFM study. Surf Sci 1999;433–435:637– 41. [10] Le Mogne T, Donnet C, Martin JM, Tonck A, Millard-Pinard N, Fayeulle S, et al. Nature of super-lubricating MoS2 physical vapor deposition coatings. J Vac Sci Tech A 1994;12:1998– 2004.