IL102440A - Oriented polycrystalline thin films and fullerene-like structures of transition metal chalcogenides - Google Patents

Description

102440/3 la e nana avpaaip p t.»wy ¾> ORIENTED POLYCRYSTALLINE THIN FILMS AND FULLERENE-LIKE STRUCTURES OF TRANSITION METAL CHALCOGENIDES \ I 102440/2 ORIENTED POLYCRYSTALLINE THIN FILMS OF TRANSITION METAL CHALCOGENIDES FIELD AND BACKGROUND OF THE INVENTION The present invention relates to specially oriented polycrystalline thin films of transition metal chalcogenides with layered structure and to methods for their fabrication and more particularly, to polycrystalline thin films of semiconducting transition metal dichalcogenides which are oriented so that the basal plane of the crystallites is exclusively or nearly exclusively parallel to the substrate surface, and to methods for their fabrication.

There has been recent interest in the preparation of polycrystalline thin films of semiconducting transition metal dichalcogenides, e.g., WS2, WSe2, MoS2 and MoSe2. Possible applications include electrochemical and photovoltaic solar cells (see H. D. Abruna and A. J. Bard, J. Electrochem. Soc, 129 ( 1982 ) 673; G. Djemal et al., Sol. Energy Mater., 5 ( 1981 ) 403 ) . Interest also exists in use of such as thin films in solid lubrication, such as in high or low temperature environments or under ultra high vacuum where liquid lubricants are not suitable (see H. Dimigen et al., Thin Solid Films, 64 ( 1979 ) 221 ) . Other possible applications involve battery cathodes (see J. Rouxel and R. A. Brec, Rev. Mater. Sci., 16 ( 1986 ) 137 ) , and catalysis (see R. R. Chianelli, Catal. Rev. Sci. Eng., 26 ( 1984 ) 361 ) . These thin films possess a layered-type structure with strong anisotropy of their mechanical and electrical properties. The chemical bonding is much stronger within the layers (covalent bonds) than between them (Van der Waals (VdW) bonds) (see J. Ά. Wilson and Ά. D. Yoffe, Adv. Phys., 18 (1969) 193). Polycrystalline thin films of transition metal chalcogenides ordinarily grow with a lamellar structure in which the basal (or VdW) planes are perpendicular to the substrate (see R. Bichsel and F. Levy, Thin Solid Films, 116 (1984) 367; P. A. Bertrand, J. Mater. Res., 4 (1989) 180). However, for photovoltaic as well as for solid lubrication applications, growth with the VdW planes parallel to the substrate is desired (see P. D. Fleischauer, Thin Solid Films, 154 (1987) 309).

Polycrystalline thin films of transition metal dichalcogenldes have been reported to be prepared mainly by sputtering the transition metal dichalcogenides (see R. Bichsel and F. Levy, Thin Solid Films, 116 (1984) 367; A. Mallouky and J. C. Bernede, Thin Solid Films, 158 (1988) 285), by electrochemical deposition (see S. Chandra and S. Sahu, J. Phys. D., 17 (1984) 2115) and by soft selenization or sulfurization as was recently reported by Jager-Waldau et al. for tungsten (see A. Jager-Waldau and E. Bucher, Thin Solid Films, 200 (1991) 157) and molybdenum (see A. Jager-Waldau et al., Thin Solid Films, 189 (1990) 339) diselenides and disulfides.

In the soft selenization method, thin sputtered tungsten or molybdenum films were reacted with selenium in a closed tube system for tens to hundreds of hours. As a result, two types of orientations were obtained, with the basal plane of the crystallites being oriented either predominantly perpendicular (hereinafter Type I) or predominantly parallel (hereinafter Type II) to the substrate surface.

Thin films of materials such as MoS2, MoSe2, MoTe2 WS2, WSe2, ZrS2, ZrSe2, HfS2, HfSe , PtS2/ ReS2, ReSe2, TiS3, ZrS3, ZrSe3, HfS3, HfSe3, TiS2, TaS2, TaSe2, NbS2, NbSe2 and NbTe2 have been studied for both their photovoltaic and tribological properties. It has been found in both cases that thin films with Van der Waals (VdW) planes parallel to the substrate (Type II) are much preferable to those with VdW planes perpendicular to the substrate (Type I). This is especially true for photovoltaic applications where Type I structures normally exhibit very poor photovoltaic activity and where even a small percentage of Type I orientation in a predominantly Type II structure can severely degrade photovoltaic performance. It is believed that Type I structures act as recombination sites and as centers for electrochemical corrosion (see H. J. Lewerenz et al., J. Am. Chem. Soc. 102 (1980) 1877).

It has heretofore been unknown how to make polycrystalline thin films of transition metal chalcogenides which are exclusively, or nearly exclusively of Type II.

There is thus a widely recognized need for, and it would be highly advantageous to have, polycrystalline thin films of semiconducting transition metal dichalcogenides, and methods for making these thin films, which will be oriented exclusively or nearly exclusively as a Type II structure.

SUMMARY OF THE INVENTION According to the present invention there is provided a method of preparing a polycrystalline thin film of a transition metal chalcogenide, or mixtures thereof, with layered structure, of a desired orientation on a substrate, comprising: (a) depositing a layer of a transition metal material or mixtures thereof on the substrate; and (b) heating the layer in a gaseous atmosphere containing one or more chalcogen materials for a time sufficient to allow the transition metal material and the chalcogen material to react and form the oriented polycrystalline thin film.

Also according to the present invention there is provided an oriented polycrystalline thin film of a transition metal chalcogenide made by the above method.

Further according to the present invention there is provided a polycrystalline layered thin film of a transition metal chalcogenide, or mixtures thereof, on a substrate, whose Van der Waals planes are exclusively, or nearly exclusively, parallel to the substrate surface.

Further yet according to the present invention there is provided a rolled-up structure substantially circular in cross -section, made up of layered transition metal chalcogenides . This structure, which has been variously termed 'microtubular · , 'nanotubular ' , 1 fullerene-like1 or 'faux fullerene* (as in Sci. Am., February 1993, p. 24), may be obtained by the method of the invention.

According to furtheu features in preferred embodiments of the invention described below, the transition metal material is a transition metal, such as tungsten, molybdenum, zirconium, hafnium, platinum, rhenium, titanium, tantalum and niobium preferably tungsten or molybdenum, or a compound containing a transition metal, while the chalcogen material is a chalcogen, such as sulfur, selenium or tellurium, preferably sulfur or selenium, or a compound containing a chalcogen.

According to still further features in the described preferred embodiments the deposition is accomplished by sputtering, by vacuum evaporation, by chemical solution deposition or by electrodeposition.

According to yet further features in the described preferred embodiments the heating is conducted under reducing conditions, preferably using a gaseous atmosphere which includes hydrogen sulfide or sulfur and forming gas.

In preferred embodiments the heating takes place at a sufficiently high temperature and at a sufficiently low sulfur concentration as to form a thin layer whose crystallites have a basal plane which is oriented exclusively, or nearly exclusively parallel to the substrate surface, where exclusively or nearly exclusively is meant to indicate more than 95% of the chains parallel to the substrate surface, preferably over 99%, most preferably over 99.9%.

The present invention successfully addresses the shortcomings of the presently known methods by providing a method which allows a thin layer to be constructed which is made up exclusively or nearly exclusively of crystallites having a basal plane which is oriented parallel to the substrate surface.

The present invention discloses the formation of highly oriented films of transition metal chalcogenides , such as WS2, WSe2/ MoS2 and MoSe2. The oriented thin films are grown by chalcogenizing thin tungsten or molybdenum films, using S, Se, Te, or their compounds.

According to the present invention, thin tungsten or molybdenum films are deposited onto a suitable substrate and then reacted at temperatures from 500 to 1,100°C in an open system under a gas flow consisting of a mixture of H2S and forming gas.

The onset of the reaction between tungsten and H2S to give WS2 thin films was found to depend on the substrate. For example, for quartz substrate it was found to be 650°C. Orientation of the WS2 crystallites could be controlled by choice of reaction temperature and sulfur concentration in the gas flow. For example, with quartz substrate Type I structure was obtained from about 500°C to about 850°C while Type II was obtained from about 950°C to about 1,100°C. Mixtures of Type I and Type II were obtained for temperatures between about 850°C and about 950°C. Low reaction temperatures (up to 900°C) and high sulfur concentrations lead to films where the Van der Waals planes are perpendicular to the substrate, while high temperatures (more than 950°C) and low sulfur concentrations result in Van der Waals planes being parallel to the substrate.

These results may be explained as being the result of competition between the reaction rate and the rate of crystallization. The importance of these results lies in the fact that the latter orientation is needed for solar cells and optimum lubrication uses, yet it is the former which has been commonly encountered in the majority of reported cases.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings wherein: FIG. 1 is a schematic depiction of Type I -and Type II orientation of layers of polycrystalline thin films on a substrate; FIG. 2a is a transmission electron microscope (TEM) view of microtubules of WS2 of a fullerene-like, or faux fullerene, structure according to the present invention; FIGs. 2b and 2c are TEM views of a fullerene-like, or faux fullerene, structure of WS2 according to the present invention; FIGs. 3 and 4 are two TEM views of MoS2 faux fullerene structures made as described in Example 7 -- a hollow structure (Figure 3) and Mo- filled structure (Figure 4) ; FIGs 5 and 6 are two X-ray diffraction spectra as described in Example 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a method of forming a highly oriented thin polycrystalline thin layer which is particularly useful in photovoltaic and in lubrication applications. Specifically, the present invention can be used to create thin layers in which the layers lie exclusively, or nearly exclusively, parallel to the plane of the substrate.

Films of MX2, where is Mo, or W and X is S, Se or Te, prepared by annealing sputter-deposited tungsten on quartz in flowing H2X or elemental X vapor exhibit different crystal textures depending on the preparation conditions. Low chalcogenizing temperatures lead to exclusively or nearly exclusively Type I texture (VdW planes vertical*) which is the texture usually obtained when films of this, or similar materials, are prepared by various methods.. Higher temperatures give Type II texture (VdW planes parallel to the substrate, which is the desirable texture in many cases) with either very small amounts (about 1%) of vertical VdW plane component ( for high chalcogen concentration in the gas flow) or no TEM-measurable amounts of this component for low chalcogen concentrations. It is believed that the phenomenon is the result of competition between the reaction rate to MX2 and the rate of crystallization of the film. A slow deposition rate of the sputtered M films leads to high amounts of Type I texture, even at 1000°C, which may be due to impurities in the slowly growing film.

In a process according to the present invention, a polycrystalline thin film of a desired orientation on a substrate is formed by first depositing a layer of a transition metal material on the substrate. Any suitable substrate may be used. Preferably, the substrate is quartz, glass or titanium. The substrate may also be tungsten or molybdenum foils.

It has been discovered that the substrate plays an Important role In determining both the temperature at which reaction between, for example, tungsten and H2S occurs and the orientation of the WS2 film. In general, the WS2 phase nucleates from the metastable WS3 amorphous phase. For glass substrates, reaction occurs at 400°C. The metastable amorphous WS3 phase Is first formed and it decomposes with the formation of the stable WS2 phase. The WS2 film exhibits predominantly, although not exclusively, Type II orientation at 500°C. For quartz substrates, reaction occurs at 650°C and the orientation is predominantly Type I below 950°C and exclusively or nearly exclusively Type II at higher temperatures . Molybdenum substrates give Type I orientation even at 1000°C and bulk tungsten (or tungsten sputtered onto tungsten) give randomly oriented WS2. Without in any way limiting the scope of the present invention, it is suggested that the difference between glass and quartz substrates is due to the stronger bonding of the W and WS2 to the quartz.

The transition metal material may be a transition metal or a compound containing a transition metal, or a mixture of transition metals and/or compounds containing a transition metal, such as tungsten, molybdenum, zirconium, hafnium, platinum, rhenium, niobium, tantalum and titanium, . Preferably, the transition metal is tungsten or molybdenum, or mixtures thereof.

Deposition of the transition metal or transition metal compound onto the substrate may be accomplished in any suitable way. These may include sputtering, vacuum evaporation, chemical solution deposition and electrodeposition.

After the deposition of the layer of transition metal material, the layer is heated in a gaseous atmosphere containing a chalcogen material which may be one or more chalcogens and/or one or more compounds containing one or more chalcogens. Preferably, the chalcogen material is sulfur or selenium or mixtures thereof.

Heating is continued at a sufficiently high temperature and for a sufficiently long time to allow the transition metal material to react with the chalcogen material and form the oriented polycrystalline thin film.

Optimal temperatures and durations of heating to be used depend on the precise nature of the reactants and on the desired orientation of the polycrystalline layers.

The heating is preferably carried out under reducing conditions. The gaseous atmosphere under which the reaction is carried out preferably includes H2X or elemental X and forming gas.

The heating takes place at a sufficiently high temperature and at a sufficiently low chalcogen concentration for the formation of a thin layer whose crystallites have a basal plane which is oriented exclusively or nearly exclusively parallel to the substrate surface. The suitable temperature ranges depend on the nature of the chalcogen and the transition metal, as well as on the substrate. Typically, the heating temperature is in the range of from about 600°C to about 1,100°C. The precise temperature ranges depend on the sμbstrate used and on whether it is desired to produce Type I or Type II structures. For example, for a quartz substrate, Type I structures are obtained at temperatures below 950°C while Type II structures are achieved at temperatures above 950°C. The suitable chalcogen concentration ranges depend on the nature of the chalcogen and the transition metal, as well as on the substrate. E.g., for sulfur, the concentration is in the range of from one or a few ppm to about 10%.

The principles and operation of a method and product according to the present invention may be better understood with reference to the examples and descriptions below, including the accompanying drawings. A number of examples are given to illustrate various aspects of the present invention. The discussion which follows concentrates on the WS2 and MoS2 systems, it being understood that this discussion is illustrative only and is not intended to limit the scope of the present invention to that system.

EXAMPLE 1 Thin films of molybdenum (about 50 nm thick) were ion beam sputtered onto quartz slides, at an argon pressure (in the chamber) of 2 x 10'4 Torr. The sputtering rate was about 10 nm min1. The samples were reacted in an open quartz furnace with forming gas (5% H2, 95% N2, 50 cm3 min-1 and residual sulfur on the walls of the quartz tube). The reaction temperature was 1000°C. After the furnace reached the desired temperature and a uniform gas flow was maintained, the samples were introduced into the hot zone of the reaction chamber, by a magnet from outside the quartz tube, for 1 hour. After reaction the samples were moved back to the cold zone of the reaction chamber, where a temperature of about 60°C was maintained.

The reaction products were characterized by X-ray diffraction (XRO) (CuKct radiation), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), Auger electron spectroscopy (AES), optical transmission spectroscopy (OTS) and sheet resistivity measurements using a four-point probe .

Thin foils for TEM study were obtained by peeling off the films from the quartz substrate by immersing in very dilute HF (about 1%) .

The analyses indicated that the resulting structure had exclusively Type II orientation with more than 99.9% of the structure being oriented parallel to the substrate.

EXAMPLE 2 A solution of ammonium thiomplybdate was prepared by saturating 50 ml of an aqueous solution of ammonium molybdate (5g) with H2S. This solution was used as an electrolyte, with a Pt anode and a Ti cathode. A constant current of 1 mA cm"2 for 20 minutes was passed through the electroplating cell, which was cooled in an ice-water bath.

The layer was analyzed by Auger electron spectroscopy and found to be MoOS2. It was annealed in 0% H2S in argon at 700°C for 30 minutes. Subsequent analysis of the annealed film showed it to be MoS2. Tne resulting structure was of mixed Type I and Type II orientation.

EXAMPLE 3 Ammonium molybdate (2g) and elemental sulfur (0.1g) were dissolved in dimethyl sulfoxide (50 ml) at 130°C. This solution was used as an electrolyte in an electrolysis cell using a carbon anode and Ti cathode. A constant current of 0.5 mA cm-2 was passed for 20 minutes at an electrolyte temperature of 130°C.

The resulting film was found, by electron diffraction, to be composed of Mo02 crystals. This film was annealed in a mixture of forming gas with 0.1% H2S at 800°C for 20 minutes, which converted it to MoS2. The resulting structure was of mixed Type I and Type II orientation.

EXAMPLE 4 Thin films of tungsten (about 50 nm thick) were ion beam sputtered onto quartz slides, at an argon pressure (in the chamber) of 2 x 10~4 Torr. The sputtering rate was about 10 nm min-1. The samples were reacted in an open quartz furnace with a flow of H2S (5-50 cm3 min"1) and forming gas (5% H2, 95% N2, 150 cm3 min"1). The reaction temperatures were controlled In the range from 500 to 1000°C. After the furnace reached the desired temperature and a uniform gas flow was maintained, the samples were introduced into the hot zone of the reaction chamber, by a magnet from outside the quartz tube, for times between 5 minutes and 5 hours. After reaction the samples were moved back to the cold zone of the reaction chamber, where a temperature of about 60°C was maintained. The reaction products were characterized as in Example 1.

The results of the analyses are described below. The tungsten layer after deposition was found by XRD as well as by TEM to be either amorphous or with a grain size of 1 nm or less. In order to study systematically the onset of the reaction, the tungsten/quartz specimens were reacted at temperatures between 500 and 1000°C for 30 minutes. EPMA measurements were performed after the reaction. The atomic concentrations of the elements were found to be in a good agreement with the WS2 stoichiometry and did not depend on the heat treatment conditions within the experimental error of this technique.

It was verified by OTS, XRD, AES and TEM that the onset reaction temperature is 650°C. At this temperature the WS2 hexagonal 2H polytype (see Powder Diffraction File, ASTM, Philadelphia, PA, card 8 237) was formed. The sheet resistivity measurements were also in agreement with the other techniques and revealed an increase of three orders of magnitude in the resistivity of the layer between 600 and 650 °C, indicating the transformation from metallic tungsten to semiconducting WS2. The AES depth profile taken from such specimens showed uniform distribution of tungsten and sulfur with negligible amounts (less than 1%) of carbon and oxygen in the bulk.

TEM observations demonstrated no change in the film mlcrostructure within the temperature range of from 650 to 900 °C, although a slight tendency toward grain growth was observed with increasing reaction temperature as can be seen in the Table below.

T(°C) Time Texture ResisXRD TEM (hrs) tivity grain grain size size (nm) (nm) 500 0.5 No rxn 2.7x10_J 650 0.5 Type I 6.1 10 750 0.5 Type I 3.1 10-13 850 0.5 Type I 2.4 13-15 1000 0.5 Type II 1.3 30 30-200 1000 3.0 Type II 1.3 100 40-350 1000 5.0 Type II 200 50-300 TEM micrograph and selected area diffraction pattern (SADP) from a specimen following reaction at 750 °C for 30 minutes clearly show the lattice image of the VdW planes, showing clear Type I orientation. The lattice spacing is shown to be approximately 6.18 Angstrom. The most intense ring on the SADP was identified with the {002} reflection as expected for Type I texture.

The XRD spectra from such specimens are in good agreement with the TEM results. XRD spectra taken from a specimen following reaction at 650°C for 30 minutes showed the {100} and {110} reflections to be at 2Θ = 32.767° and 2Θ = 58/427°, respectively, which is consistent with Type I orientation.

However, in addition to the Type II orientation, also observable were small amounts (about 1%) of Type I oriented grains. The SADP revealed the absence of almost the entire {002} diffraction ring as expected for such a growth type. The commonly occurring moire patterns indicate that most of the microcrystallites with the VdW parallel orientation are mutually rotated about the c axis, and that the thicknesses of the crystallites are typically much less than the film thickness, which were about 200 nm.

XRD spectra of such specimens (1000°C, 30 minutes) showed only the {001} reflections where 1 = 2n corresponding to the 2H polytype of WS2) and clearly indicated, in good agreement with the TE observation, nearly exclusively Type II texture. In this case the intensity of the {002} peak was a few orders of magnitude stronger than for the specimens after reaction at lower temperatures .

It was found that the sulfur supply rate is extremely important in achieving exclusively Type II orientation. Best results were obtained when the samples were introduced into the reaction chamber for 30 minutes at 1000°C in the presence of flowing forming gas only, without the direct addition of H2S. After this treatment, it was found that the tungsten fully reacted with residual sulfur present in the reaction chamber to give WS2. Moreover, the WS2 was found by XRD and TEM to feature exclusively Type II orientation. No crystallites with Type I orientation could be observed in either the image or the SADP. The grain size was not uniform with some grains being as large as 1 um, but most being noticeably smaller (about 100 nm) .

TEM micrographs from a specimen reacted with residual sulfur as described above, followed by 30 minutes reaction at 1000°C with the usual conditions of H2S and forming gas flow showed an increasing number of the large grains (about 1 um) . This is of importance for photovoltaic applications, where grain size should be as large as possible.

The table above summarizes the orientation, sheet resistivity, and grain sizes as a function of reaction conditions. In addition to the TEM measurement, the grain size was also estimated by measuring the full width at half-maximum of the XRD diffraction peak compared with a single-crystal specimen.

The results described above show that thin films of transition metal dichalcogenides grow in two preferential orientations: Type I (VdW planes perpendicular to the substrate), which is the more common growth pattern, and Type II (VdW planes parallel to the substrate) .

It has been discovered that four main factors have an important influence on the texture of the tungsten disulfide thin films: (1) reaction temperature, (2) rate of the sulfur supply, (3) the deposition rate of the sputtered transition metal film, as it relates to the purity of the film and (4) the nature of the substrate.

The interaction between thin films of tungsten and H2S on quartz begins at 650°C. The interaction is very quick and takes only a few minutes to be completed.

It was demonstrated that, at reaction temperatures lower than 1000°C, the orientation of the WS2 grains was always of Type I, while at 1000°C most of the specimens exhibited Type II texture. Without in any way limiting the scope of the present invention, it is believed that this phenomenon results from the kinetic conditions controlling the process. In spite of the orientation difference that exists in the two temperature regions, there is no evidence that the reaction at 1000°C (or above) starts with Type I orientation and then transforms to Type II orientation during the process. It was found that when the conditions are not ideal for Type II growth, such as low reaction temperatures or low rates of tungsten deposition, the orientation begins and remains Type I. Further reaction time or post-annealing treatments leads to grain growth but have no effect on the texture.

In contrast with work reported in the literature, which in some cases reported predominantly Type II orientation (see A. Jager-Waldau and E. Bucher, Thin Solid Films, 200 (1991) 157; A. Jager-Waldau et al., ICSFS-6, 29 June-3 July, 1992, Paris), layers made according to the present invention can be made to contain no Type I oriented grains whatsoever. This difference is of great importance, since it is known that even very low concentrations of Type I oriented grains can severely degrade photovoltaic performance (see W. Kautek et al., Ber. Bunsenges. Phys. Chem., 83 (1979) 1000). Also, in contrast with other reported methods, the method described here is readily amenable to scale-up and can therefore be used to deposit films of large areas.

The contamination of the slowly growing tungsten film at low deposition rates is a very important factor affecting the texture. As mentioned above, in this case Type I rather than Type II growth occurs. It is believed that this behavior can be attributed to oxygen and/or water vapor adsorption which is inversely proportional to the deposition rate. If the tungsten film (deposited under normal conditions, i.e. about 10 nm min"1 ) is first oxidized by heating in air, and then sulfided, there is a greater tendency to Type I orientation compared with unoxidized tungsten.

Without in any way limiting the scope of the present invention, it is believed that the results described above may be explained as follows.

Since high sulfur concentration is associated with Type I growth and low sulfur concentration is conducive of Type II growth, it is suggested that there is competition between diffusion-crystallization in the WS2 lattice and the reaction of the compound formation. If the reaction is kept at a low rate, as through a low concentration of sulfur, and the diffusion-crystallization processes are speeded up, as through use of high temperatures, the layer will have more time to grow in a Type II orientation, since it is the more energetically favorable orientation, since the reaction is slower than the crystallization.

In certain applications it may be desirable to form superstructures which feature various chemical species. For example, one can form a number of layers wherein the chemical makeup of one or more layers differs from that of an adjoining layer or group of layers. Similarly, each layer may be made up of a single chemical species or may include a mixture of a number of such species.

EXAMPLE 5 A thin film of tungsten was deposited on quartz as in Example 4. This film was annealed at 1000°C for 70 minutes in an argon flow with residual sulfur from the walls of the quartz reaction tube followed by 60 minutes in a flow of pure H2. While the film was composed predominantly of flat crystals oriented with the Van der Waals planes parallel to the substrate, some tubes and closed polyhedra (faux fullerenes) of WS2 were also formed. These tubes were several hundred nm long and several tens of nm in diameter, and were formed of many concentric WS2 tubes as shown in Figure 2a. Figures 2b and 2c show faux fullerenes of WS2.

It has thus been found that the WS2 crystals can, in some cases, curve to form essentially concentric structures of substantially cylindrical shape. These structures have a decided advantage for photovoltaic cell purposes, in that the amount of exposed edge face is much less than for the equivalent flat crystals. Furthermore, we sometimes observe closing of the edge of the tube by what appear to 21 102440/3 be Van der waals faces, reducing to essentially zero the exposed edge planes. Such structures may be useful for other purposes, apart from photovoltaic cells, because of their nanoscale structure. Thus, they could, in principle, be used as quantum wires and modified by introducing foreign atoms or molecules either into the Van der Waals gaps between layers (intercalation) or into the hollow center of the tube.

EXAMPLE 6 Fullerene-like structures, or faux fullerenes, based on WS2 were obtained in large quantities and with various structures by the following method. A thin film of W (100 - 300 Angstroms) was deposited on a quartz substrate by electron beam or ion beam deposition. The sample was introduced into the furnace where it is fired for approximately one hour at 950 to 1050°C in the presence of air. This leads to the creation of a stoichiometric trioxide layer as confirmed by various measurements. Subsequently, the quartz tube of the furnace was closed and a stream of H2S (1 - 5 ml/min) and forming gas (5% H2, 95% N2, 50 - 150 ml/min) were passed through the system for one to two hours, usually without altering the temperature. After the samples were allowed to revert to ambient conditions they were taken for various analyses, of which transmission electron microscopy (TEM) with imaging and selected area electron diffraction (SAED) were the most relevant for the determination of the fullerene-like 21a 102440/2 structures, or faux fullerenes. Polyhedra were found to be most abundant when the sample was fired at 1050°C.

EXAMPLE 7 Fullerene-like structures, or faux fullerenes, based on MoS2 were produced by using a similar method to that described in Example 6 above. However, since Mo03 is much more volatile than W03, it was necessary to perform the annealing at lower temperatures (950°C) , and hence the structures were less abundant, and possibly different in growth mechanism, from those obtained with WS . Two typical structures were obtained -- (1) round-shaped polyhedra consisting of a very few layers (2 to 5) and a dark core which is suspected of being of metallic nature, as shown in Figure 3, and (2) hollow structures with many (>10) layers of MoS2, as shown in Figure 4. The fringe contrast in the polyhedra was preserved on tilting the sample, while the fringe contrast of nearby platelets was lost upon tilting the sample, which is a clear indication in support of the close nature of the polyhedra. This is confirmed also by SAED which showed the diffraction pattern of two perpendicular planes at the same time.

EXAMPLE 8 Oriented MoS2 films were obtained on a quartz substrate by the following procedure. A 240 - 550 Angstrom Mo film was deposited on pre-cleaned quartz substrate by electron beam evaporation. The sample was fired (pre -annealed) for 21b 102440/2 one hour in the atmosphere of the forming gas at 960 -1000°C. X-ray powder diffraction spectra showed that under these conditions the Mo recrystallized with a typical grain size of one micrometer. Subsequently H2S was added to the reaction mixture and annealing continued for one to three hours. At 960°C the orientation of the MoS2 crystallites was almost exclusively Type II structures, as shown in Figure 5. Firing at 1000°C without pre-annealing lead to preferred Type I orientation, as shown in Figure 6.

While the invention has been described with respect to several preferred embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims (44)

22 102440/3 WHAT IS CLAIMED IS:

1. . A method of preparing a polycrystalline thin film of a transition metal chalcogenide, or mixtures of such chalcogenides, with layered structure, of a desired orientation on a substrate, comprising: (a) depositing a layer of a transition metal material or mixtures of such materials on the substrate; and (b) heating said layer in an open system under a gas flow containing one or more chalcogen materials for a time sufficient to allow said transition metal material and said chalcogen material to react and form the oriented polycrystalline thin film.

2. A method as in claim 1 wherein said transition metal material is one or more transition metals.

3. A method as in claim 1 wherein said transition metal material is one or more compounds containing one or more transition metals.

4. A method as in claim 1 wherein said chalcogen material is a chalcogen or a mixture of chalcogens.

5. A method as in claim 1 wherein said chalcogen material is one or more compounds containing chalcogens. 23

6. A method as in any of claims 1-3 wherein said transition metal is tungsten and/or molybdenum.

7. A method as in any of claims 1 , 4 and 5 wherein said chalcogen is sulfur and/or selenium.

8. A method as in and of claims 1 to 7 wherein said depositing is accomplished by sputtering.

9. . A method as in any of claims 1 to 7 wherein said depositing is accomplished by vacuum evaporation.

10. A method as in any of claims 1 to 7 wherein said depositing is accomplished by electrodeposition.

11. 1 . A method as in any of claims 1 to 7 wherein said depositing is accomplished by chemical solution deposition.

12. A method as in any of claims 1 to 11 wherein the substrate is quartz.

13. A method as in any of claims 1 to 11 wherein the substrate is glass.

14. A method as in any of claims 1 to 11 wherein the substrate is titanium. 24 102440/2

15. A method as in any of claims 1 to 14 wherein said heating is conducted under reducing conditions.

16. A method as in any of claims 1 to 15 wherein said gaseous atmosphere includes hydrogen sulfide and forming gas .

17. A method as in claim 16 wherein said gaseous atmosphere includes sulfur and/or selenium and forming gas.

18. A method as in any of claims 1 to 17 wherein said heating takes place at a sufficiently high temperature and at a sufficiently low sulfur concentration in said gaseous atmosphere as to form a thin layer in which over 95% of the crystallites have a basal plane which is oriented parallel to the substrate surface.

19. A method as in any of claims 1 to 17 wherein said heating takes place at a sufficiently high temperature and at a sufficiently low sulfur concentration in said gaseous atmosphere as to form a thin layer in which over 99% of the crystallites have a basal plane which is oriented parallel to the substrate surface. 25 102440/2

20. A method as in any of claims 1 to 17 wherein said heating takes place at a sufficiently high temperature and at a sufficiently low sulfur concentration in said gaseous atmosphere as to form a thin layer in which over 99.9% of the crystallites have a basal plane which is oriented parallel to the substrate surface.

21. A method as in any of claims 1 to 17 wherein said heating takes place at a sufficiently low temperature and at a sufficiently high sulfur concentration in said gaseous atmosphere as to form a thin layer in which over 95% of the crystallites have a basal plane which is oriented perpendicular to the substrate surface.

22. A method as in any of claims 1 to 17 wherein said heating takes place at a sufficiently low temperature and at a sufficiently high sulfur concentration in said gaseous atmosphere as to form a thin layer in which over 99% of the crystallites have a basal plane which is oriented perpendicular to the substrate surface.

23. A method as in any of claims 1 to 17 wherein said heating takes place at a sufficiently low temperature and at a sufficiently high sulfur concentration in said gaseous atmosphere as to form a thin layer in which over 99.9% of the crystallites have a basal plane which, is oriented perpendicular to the substrate surface. 26 102440/2

24. A method as in any of claims 18 to 20 wherein said substrate is quartz and wherein said temperature is between about 950°C and about 1,100°C.

25. A method as in any of claims 21 to 23 wherein said substrate is quartz and wherein said temperature is between about 500°C and about 850°C.

26. A method as in any of claims 18 to 20 wherein said sulfur concentration is between about 0.0001% and about 10%.

27. An oriented polycrystalline thin film of a transition metal chalcogenide formed by the method described in any of claims 1-26.

28. A polycrystalline layered thin film of a transition metal chalcogenide, or mixtures thereof, on a substrate, in which over 95% of the Van der Waals planes are parallel to the substrate surface.

29. A polycrystalline layered thin film of a transition metal chalcogenide, or mixtures of such, on a substrate, in which over 99% of the Van der Waals planes are parallel to the substrate surface. 27 102440/3

30. A polycrystalline layered thin film of a transition metal chalcogenide, or mixtures of such, on a substrate, in which over 99.9% of the Van der Waals planes are parallel to the substrate surface.

31. A thin film as in any of claims 28 to 30 wherein said transition metal is tungsten and/or molybdenum.

32. A thin film as in any of claims 28 to 30 wherein said chalcogen is sulfur or selenium.

33. A thin film as in any of claims 28 to 32 wherein said substrate is quartz.

34. A thin film as in any of claims 28 to 32 wherein said substrate is glass.

35. A thin film as in any of claims 28 to 32 wherein said substrate is titanium.

36. A faux fullerene structure substantially circular in cross -section, made up of layered transition metal chalcogenides . 28 102440/1

37. A faux fullerene or fullerene-like structure according to claim 36 being round-shaped polyhedra or multilayered hollow structures.

38. 3 8. A fullerene-like structure according to claim 37 being of nanotubular shape.

39. A method for the preparation of fullerene-like structures of a transition metal chalcogenide, or mixtures thereof, with layered structure, on a substrate, comprising: (a) depositing a thin layer of a transition metal material or mixtures thereof on the substrate; (b) heating said layer in the presence of air at a temperature of 950- 1050'C, thus obtaining a stoichiometric metal oxide layer; and (c) heating said metal oxide layer in an open system under a gas flow atmosphere containing one or more chalcogen materials for a time sufficient to allow said transition metal oxide material and said chalcogen material to react and form the desired fullerene-like structure.

40. A method according to claim 39 wherein the transition metal material is a transition metal and the chalcogen material is a sulfur compound.

41. A method according to claim 40 wherein the transition metal is tungsten and/or molybdenum and the trioxide layer obtained in step (b) is heated with H2S in step (c), thus obtaining fullerene-like structures based on WS2 and/or MoS2. 29 102440/1

42. A method according to claims 39-41 wherein the fullerene-like structure is round-shaped polyhedra consisting of few layers and a dark core or a multilayered hollow structure.

43. A method according to claims 39-42 wherein said substrate is quartz.

44. A faux-fullerene or fullerene-like structure of a transition metal chalcogenide, or mixtures thereof, obtained by a method as in claims 39-43. / Dr. Mark M. Friedman Advocate, Patent attorney Beit Samueloff Haomanim 7 Tel Aviv 67897