IT201800002349A1 - METHOD FOR THE PRODUCTION OF THIN FILMS OF TRANSITION METAL DICALCOGENIDE - Google Patents

Description

Description of the industrial invention entitled:

"METHOD FOR THE PRODUCTION OF THIN FILMS OF TRANSITION METAL DICALCOGENIDE"

FIELD OF THE INVENTION

The present invention relates to a method for producing transition metal dicalcogenide films, having a thickness of a single layer or a few layers of material, in particular with geometry defined by direct writing on a support.

STATE OF THE TECHNIQUE

In recent years, the dicalcogenides of transition metals have been the subject of intensive research for the electronic properties deriving from their layered two-dimensional crystalline structure, similar to that of graphenes.

In particular, it has been observed that while these dicalcogenides in massive form have an indirect gap between the valence and conduction bands, thin films of the same materials, with a thickness of a few layers or at the limit of a monolayer, have a direct gap, meaning with "monolayer" a layer corresponding to the thickness of an unit cell of the material of interest.

The change in the structure of the bands resulting from the reduction in thickness, from massive material to thin film, affects in particular the optical properties of the materials, and allows the radiative recombination between electrons and holes making the material suitable for the production of devices such as light diodes and lasers ; in the same way, it allows a high efficiency of generation of hole-electron pairs following absorption of light radiation, making the material suitable for use in photovoltaic cells. Other applications have been hypothesized and experimentally verified in the fields of electronics, energy and sensors.

Dicalcogenides useful for these applications are for example the disulfides of molybdenum, tungsten or niobium (respectively MoS2, WS2 and NbS2) or the selenides of the same metals (MoSe2, WSe2 and NbSe2). Particularly interesting and extensively studied is molybdenum disulfide and research has mainly focused on this material.

MoS2 thin films have been produced according to a wide variety of techniques. For example, these thin films have been produced by exfoliation from the massive material, both mechanically, for example by removing surface layers of the material with the use of adhesive tapes; either chemically, for example with methods of exfoliation through intercalation of alkaline or alkaline-earth metals (method followed in patent application WO 2015/102191 A1) or exfoliation in solution. These methods have the disadvantage of not allowing a viable and effective integration in an industrial process based on these materials.

An alternative approach is the direct synthesis of the thin film on a substrate, which can be accomplished according to various methods.

A first method is Chemical Vapor Deposition (CVD), as described for example in the article "Atomic-Scale Structure of Single-Layer MoS2 Nanoclusters", S. Helveg et al, Phys. Rev. Lett. 2000, 84, 951−954.

A second possibility is the sulfurization of molybdenum oxides, as described for example in the article "Two-Dimensional Nanosheet Crystals", J. Seo et al, Angew. Chem., Int. Ed. 2007, 46, 8828−8831.

However, these methods are complex and often use hazardous process gases.

Finally, another method for the direct synthesis of thin films of dicalcogenides involves the deposition on a substrate of a precursor containing Mo and S atoms, and subsequent thermolysis of the precursor.

The article "MoSx thin films by thermolysis of a single-source precursor", J. Pütz et al, Journal of Sol-Gel Science and Technology 19, 821–824, 2000 describes the production of thin films of MoS2 by spin coating on a substrate of a solution of diammonium tetratiomolybdate ((NH4) 2MoS4) in various amine solvents, drying of the liquid film in an oven, and decomposition of the precursor under reduced pressure and under a reducing atmosphere containing hydrogen at temperatures between 300 and 800 ° C.

The article "Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates", K.-K. Liu et al, Nano Letters, 2012, 12 (3), 1538-1544 describes a modification of the method of the previous article, in which the film, after its preparation, is subjected to an annealing treatment (or rather, of " annealing ”, term commonly used in the sector) at temperatures between 500 and 1000 ° C first in a mixed Ar / H2 atmosphere, and then in an Ar atmosphere containing sulfur vapors; the purpose of the latter treatment is to "repair" defects in the MoS2 structure, due to the presence of carbon impurities, formed during the initial production of the film.

The methods that start from precursor compounds containing Mo and S are advantageous compared to the previous ones, but must be carried out in controlled atmospheres, in particular reducing ones, because the intermediate species that are formed in thermolysis are easily oxidizable, and in the presence of oxygen or compounds oxygenated gases would give rise to oxides thus altering the composition of the film with respect to the desired one.

The object of the present invention is to provide a method for producing transition metal dicalcogenide films having a thickness of from one to a few atomic layers by means of precursor photo-thermolysis, which overcomes the problems of the known art avoiding the need to carry out the transformation from precursor to dicalcogenide in a controlled atmosphere.

SUMMARY OF THE INVENTION

According to the present invention, this object is achieved with a process of producing a layer of a dicalcogenide of a transition metal having a thickness of from one to a few elementary cells by decomposition of a precursor compound, which includes the following steps:

a) preparation of a solution consisting of a solvent and a precursor compound containing the desired metal and chalcogen element; b) deposition of the solution of step a) on a substrate by spincoating or by immersion of the substrate in the solution, with the formation of a film of said solution on the substrate;

c) evaporation of the solvent of the solution film obtained in step b) in a controlled atmosphere at a temperature between 80 and 225 ° C, with the formation of a dry deposit of precursor on the substrate;

d) decomposition of the dry deposit of precursor obtained in step c) to form the desired metal dicalcogenide;

characterized in that step d) is carried out by laser irradiation of said dry precursor deposit.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described with reference to the Figures, in which:

- Fig. 1 shows a photomicrograph of a sample of the invention, which highlights the precursor area treated with laser;

- Fig.2 shows the Raman spectrum of a sample of MoS2 obtained according to the invention and of a sample of the same material obtained according to a method of the known technique;

- Fig. 3 shows three Raman spectra for the material, MoS2, bulk (A), for a tri-layer (B) and for a bilayer (C), the latter two obtained according to the invention;

- Fig. 4 shows a photomicrograph of a sample of the invention treated with UV laser which highlights the precursor area treated with laser;

- Fig. 5 shows the Raman spectrum of a WS2 sample obtained according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the description and claims, the following terms are meant as specified:

- "chalcogen": an element of group 16 of the periodic table, with the exception of oxygen;

- "dicalcogenide": a compound between a chalcogen and a transition metal, in which the atomic ratio between said chalcogen and metal is 2: 1.

The steps from a) to c) of the process are carried out with methods known in the field, therefore they are only briefly described below.

In step a) of the process of the invention, the solution of the precursor of the final compound of interest is produced. The solution is prepared by dissolving a precursor in a suitable solvent, with concentration values generally between 0.5 and 2% w / v in the case of a solution to be used by immersing the substrate in the same ("dip-coating" technique) , and between 0.1 and 0.2% w / v in the case of a solution to be used in the spin-coating technique; concentrations indicated as “% w / v” indicate the weight in grams of solute per 100 mL of solvent. The precursors are generally alkylammonium salts, or more commonly ammonium salts, of anions consisting of sulfur or selenium and the metal of interest, such as the compounds (NH4) 2MoS4, (NH4) 2WS4, (NH4) 2MoSe4 and (NH4 ) 2WSe4. The preferred solvents for the preparation of the solutions are dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and ethylene glycol.

In step b), the substrate on which the dicalcogenide layer is to be produced is coated with a film of the solution described above.

The substrate is generally an alumina or silicon wafer coated with a thermal silicon oxide (i.e. a thin layer of silicon oxide formed on the surface of a silicon substrate by heat treatment of the latter in an oxidizing atmosphere).

The deposition of the solution on the substrate is achieved by spincoating or by immersion of the substrate in the solution. In the case of spin-coating, the substrate typically has a circular shape; at the center of the substrate a quantity of solution of about 10 µL is deposited, and the substrate is then put in rotation with a speed between 3000 and 4000 rpm for a time between 30 and 120 seconds. In the case of dip-coating, the substrate is immersed vertically in the solution and then extracted from it, vertically or with a variable angle up to 30 ° with respect to the vertical, with a speed between 0.2 and 0.5 mm / sec, possibly with multiple immersion / extraction cycles.

In step c) of the process, the solution film thus obtained is dried in a controlled atmosphere, for example of nitrogen, at a temperature of between 80 and 225 ° C; the operation normally takes about 5 minutes. In this way, a thin, dry deposit of only the precursor on the substrate is obtained.

Characterizing of the process of the invention is step d), in which the decomposition of the precursor film obtained in step c) is carried out with the formation of the desired metal dicalcogenide.

Unlike the decomposition methods of the known art, according to the invention this step is carried out by laser irradiation. It has not yet been clarified whether laser decomposition occurs on the basis of a thermolysis or photolysis mechanism.

However, the inventors have observed that by operating according to the invention, i.e. by carrying out step d) by laser irradiation, layers of dicalcogenides having the desired properties are obtained by operating in air, thus avoiding the need to work under a hydrogen atmosphere (or other controlled atmospheres ).

The decomposition of the precursor to form the corresponding dicalcogenide occurs selectively in the area irradiated with the laser. With the method of the invention it is therefore possible to convert the entire precursor by scanning the laser over the entire surface, for example by operating in sequence on parallel lines.

However, the method provides an alternative opportunity, not accessible through the thermal decomposition methods of the known art, which is to obtain shaped thin layers (or, with a common term in the sector, "patterned"), having a desired geometry on the substrate. In this second operating mode, after having carried out the selective formation of the dicalcogenide in the desired zones, it is possible to remove the unconverted precursor by dissolution, generally with the same solvents used in step a), in which the metal dicalcogenides object of the invention are insoluble , thus obtaining as a result of the process a substrate on which only a deposit of predetermined geometry of the dicalcogenide of interest is present.

To obtain the decomposition of the precursor, the laser must have a minimum power density that can vary depending on the excitation wavelength. Typically for an excitation at 2.3 eV, the minimum power density is of the order of 10 <5> W / cm <2>.

The invention will be further illustrated by the following examples. For the realization of the examples and characterizations, the following tools and equipment were used:

- Spin-coater SCS Specialty Coating Systems 6708D;

- 532 nm laser beam: Lightwave Electronics 142 H-532-200;

- Optical microscope with Olympus MPlan N 100x / 0.90NA objective;

- Jobin-Yvon T6400 Raman spectrometer.

EXAMPLE 1

Three molybdenum disulfide samples were prepared according to the methods of the invention.

A SiO2 substrate (200 nm layer of SiO2 grown thermally on silicon) was previously washed at 80 ° C for 40 minutes with the so-called "piranha solution", consisting of 7 parts by volume of H2SO4 (purity 99% in weight) and 3 parts by volume of H2O2 (solution in water at 30% by weight); the substrate was then rinsed in deionized water for 5 minutes and then with isopropanol for 5 minutes; isopropanol was finally removed from the substrate by nitrogen flow.

The precursor solution was prepared separately by dissolving 11.0 mg of ammonium tetratiomolybdate, (NH4) 2MoS4 (Carlo Erba, catalog no. 15060-55-6, purity 99.99%), in 5.526 g of ethanol. The solution was heated under magnetic stirring in an oil bath at 70 ° C for 20 minutes and then filtered using a 0.2 µm filter.

The precursor solution was deposited on the substrates by spincoating, inside a chamber with a controlled inert atmosphere (nitrogen) and free of humidity. 5 µL of solution were deposited on each substrate; spin-coating was performed at 200 rpm for 30 s; the ethanol was then left to evaporate.

Each of the samples thus obtained was irradiated in the air with a laser (light with a wavelength of 532 nm) in a continuous mode, with a power of 0.6 mW, focusing the radiation with an objective with a numerical aperture of 0.75. Areas of about 10 × 20 m <2> were treated, obtaining samples of dicalcogenide layers as shown in the photomicrograph in Fig.1, where the white bar is 2 µm long.

The resulting MoS2 layer was characterized by Raman spectroscopy; the spectrum is shown in Fig. 2, spectrum A, together with a spectrum of a sample obtained with the exfoliation technique, spectrum B, confirming that the sample obtained is made up of the desired compound.

As known in the sector (see for example the article by K.-K. Liu et al cited), it is possible to measure the number of layers (elementary cells) of the MoS2 film obtained by measuring the energy difference between the maxima of the two characteristic peaks of the Raman spectrum of the material, one centered between about 380 and 382 cm <-1> and the other between about 404 and 406 cm <-1>; Fig. 3 reports three spectra for the bulk material (A), for a tri-layer (B) and for a bilayer (C).

EXAMPLE 2

MoS2 samples were made with a process identical to that described in the previous example with the exception of the laser. In fact, a pulsed UV laser was used (= 351 nm and spot of about 60 m in diameter) LCS-DTL368QT (LASER EXPORT) used at 3 kHz frequency (pulse duration 6.6 ns). The size of the spot allowed a larger area of the precursor to pass through with a single irradiation; Fig. 4 shows two spots of the UV laser on the precursor. Raman spectra confirm the MoS2 transformation of the precursor.

EXAMPLE 3

The procedure of Example 1 was repeated, with the only difference that ammonium tetra-thio-tungstate, (NH4) 2WS4 (Sigma Aldirch, catalog no. 336734, purity 99.9 +%) was used as the precursor, and the resulting film consists of WS2.

The sample obtained was characterized by Raman spectroscopy; the resulting spectrum is shown in Fig. 5; this spectrum, by comparison with literature data, confirms that the sample obtained is WS2.

Claims (10)

CLAIMS 1. Process of producing a layer of a dicalcogenide of a transition metal of thickness from one to a few elementary cells by decomposition of a precursor compound, which includes the following steps: a) preparation of a solution consisting of a solvent and a precursor compound containing the desired metal and chalcogen element; b) deposition of the solution of step a) on a substrate by spincoating or by immersion of the substrate in the solution, with the formation of a film of said solution on the substrate; c) evaporation of the solvent of the solution film obtained in step b) in a controlled atmosphere at a temperature between 80 and 225 ° C, with the formation of a dry deposit of precursor on the substrate; d) decomposing the dry deposit of precursor obtained in step c) to form the desired metal dicalcogenide; characterized in that step d) is carried out by laser irradiation of said dry deposit of precursor. 2. Process according to claim 1, wherein the solution of step a) consists of a precursor selected from (NH4) 2MoS4, (NH4) 2WS4, (NH4) 2MoSe4 and (NH4) 2WSe4 and a solvent selected from dimethylformamide (DMF ), N-methyl-2-pyrrolidone (NMP), and ethylene glycol. Process according to any one of the preceding claims, wherein the substrate used in step b) is a wafer of a material selected from alumina and silicon coated with a thermal silicon oxide. 4. Process according to any one of the preceding claims, in which the deposition of the solution of step b) is carried out by means of the spin-coating technique, depositing on the substrate a precursor solution having a concentration between 0.1 and 0.2% p / v and rotating the substrate with a speed between 3000 and 4000 rpm for a time between 30 and 120 seconds. 5. Process according to any one of claims 1 to 3, in which the deposition of the solution of step b) is carried out by immersing the substrate in a precursor solution having a concentration between 0.5 and 2% w / v, vertically or with a variable angle up to 30 ° with respect to the vertical, with a speed between 0.2 and 0.5 mm / second. 6. Process according to any of the preceding claims, in which the laser irradiation of step d) is carried out in air. 7. Process according to claim 1, wherein said irradiation is carried out on the whole precursor deposit surface. 8. Process according to any one of claims 1 to 6, wherein said irradiation is selectively carried out on some areas of the precursor deposit, obtaining a shaped layer of dicalcogenide of the transition metal. Process according to claim 8 wherein, after carrying out step d), the precursor not converted into dicalcogenide of the transition metal is removed by dissolution with a solvent. 10. Layer of a dicalcogenide of a transition metal having a thickness from one to a few elementary cells, obtained by the process of any one of the preceding claims.