Transition metal dichalcogenides with multiple ferroic orders

Abstract

The present development corresponds to a material of the transition metal dichalcogenides family that possesses either more than one ferroic order, such as ferroelectricity and ferromagnetism, or only one ferroic order with tendency to a second one, wherein these properties are observable at room temperature. Particularly, present development provides a room temperature multiferroic material of Formula (I): wherein M A , M B , X A , X B , X c , z, y B and y C are as defined here and wherein the material is characterized by a layered crystalline structure made up of sub-nanometer layers..

Description

[0001] TRANSITION METAL DICHALCOGENIDES WITH MULTIPLE FERROIC ORDERS

[0002] FIELD OF THE INVENTION

[0003] The material of the present invention is related to the field of room temperature multiferroic materials useful in spintronics, data storage, and actuators, among others. In particular, the invention relates to a material of the transition metal dichalcogenides family that possesses either more than one ferroic order, such as ferroelectricity and ferromagnetism, or only one ferroic order with tendency to a second one, for example, ferromagnetism and large piezoelectricity, or conversely, ferroelectricity and saturating paramagnetism. These properties are observable at room temperature. It also encompasses the methods of preparing and characterizing the same.

[0004] BACKGROUND OF THE INVENTION

[0005] Multiferroics are materials that simultaneously exhibit different ferroic orders, such as ferroelectricity, ferromagnetism, or ferroelasticity. The coupling among these distinct ordered states allows for control over one order parameter by adjusting parameters unrelated to its conjugate variable. For instance, magnetoelectric coupling enables manipulation of magnetization through an electric field or, conversely, electric polarization through a magnetic field some in ferromagnetic / ferroelectric heterostructures and multiferroics. These materials find applications in spintronics, data storage, actuators, and more, with advancements already achieved. However, multiferroicity remains a rare property in materials.

[0006] Historically, these phenomena have primarily been observed in three-dimensional complex oxides and perovskites, such as CnCh, YMnCh, and BiFeCh, among others, as well as in heterostructures involving thin films of ferroelectric and ferromagnetic materials (Hanamura and Tanabe 2006. Phase transitions and second-harmonics of ferroelectric and antiferromagnetic RMnOs. Phase Transit. 79: 957-971; Wang et al., 2003. Epitaxial BiFeOs multiferroic thin film heterostructures. Science 299: 1719-1722; Ramesh and Spaldin 2007. Multiferroics: progress and prospects in thin films. Nat. Mater. 6: 21-29). The three-dimensional nature of the crystal structure of these compounds presents challenges for their integration into nanoscale multiferroic devices. Achieving nanostructuring demands advanced fabrication techniques and can induce quantum confinement effects that often diminish multiferroic properties. Therefore, multiferroics possessing an intrinsic two-dimensional crystalline structure are highly sought after for developing cost-effective and durable nanodevices with robust multiferroic properties.

[0007] However, discovering new and improved multiferroic materials poses challenges. Ferroelectrics, which require insulating properties to preserve electric polarization (unlike metals where free charges screen this effect), and ferromagnets, predominantly metallic, necessitate innovative approaches. One effective strategy involves chemical doping of either ferroelectric or ferromagnetic hosts. For example, substituting transition-metal ions in ferroelectric materials like LiNbOs or ZnO induces magnetic order (Chen, et al., 2011. Strong d-d electron interaction inducing ferromagnetism in Mn-doped LiNbCf. Thin SolidFilms 520:764-768; Pan et al., 2008. Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films. Mater. Sci. Eng. R: Rep. 62 1-35). Similarly, doping complex oxide multiferroics has proven successful in optimizing their properties.

[0008] Recent advancements have been made in the realization and study of intrinsically 2D- multiferroics. Examples include transition metal halide compounds like Nib (Song et al., 2022. Evidence for a single-layer van der Waals multiferroic. Nature. 602:601-605) and CrE (Zhao et al., 2018. Surface vacancy-induced switchable electric polarization and enhanced ferromagnetism in monolayer metal trihalides. Nano Lett. 18: 2943-2949: Lyu, et al., 2022. Two-dimensional intercalating multiferroics with strong magnetoelectric coupling. J. Phys. Chem. Lett. 13: 11405-11412), MXene Ti3C2Tx in p-doped SnSe (Tahir et al., 2022. First observation on emergence of strong room-temperature ferroelectricity and multiferroicity in 2DTi3C2Txfree-standing MXene film. RSC Adv. 12:24571-24578: Tahir et al., 2023. Multiferroic and ferroelectric phases revealed in 2D Ti3C2TxMXene film for high performance resistive data storage devices. NP J 2D Mater. Appl. 7: 1-8), and doped I Ses (Du et al., 2022. Two-dimensional multiferroic material of metallic p- doped SnSe. Nat. Commun. 13:6130).

[0009] In recent years, individual ferroic orders have been experimentally observed at room temperature for some members of the transition metal dichalcogenides (TMDs), the most widely studied family of 2D compounds. Ferromagnetism has been achieved in M0S2 and WS2 through defect engineering, and in WSe2 via chemical doping of the transition metal site, among other methods. Ferroelectricity has been reported in bulk-WTe2, while WSe2 and M0S2 have been identified as piezoelectric for odd numbers of layers. On the other hand, ternary material of WSe2(i-x)Te2x (x = 0-1) (Yu et al., 2017. Metal- Semiconductor Phase-Transition in WSe2(i-x)Te2x Monolayer. Adv. Mater. 29: 1-8), or WTe2-xSex(0 < x < 0.8) (Kanchanavatee et al., 2022. Phase transitions and suppression of magnetoresistance in WTe2-xSexsystem. J. Phys.: Condens. Matter. 34:435403) exhibit two distinct structures wherein, 2H and 1 Td, corresponding to semiconductor and metal, respectively.

[0010] However, achieving multiferroicity in the TMDs has remained elusive until now, despite extensive theoretical and experimental exploration. For instance, simultaneous ferromagnetism and ferroelectricity in TMDs have not been experimentally confirmed to date, nor even the presence of one of these ferroic orders accompanied by the presence of free dipoles moments associated to a second type of order (for example, ferromagnetism accompanied by large piezoelectricity). It is therefore desirable to provide a novel multiferroic material that seeks to address at least one of the problems described hereinabove, or at least to provide an alternative.

[0011] SUMMARY OF INVENTION

[0012] Present invention provides a room temperature multiferroic material of Formula (I): wherein:

[0013] MA is a transition metal element;

[0014] MB is absent or a transition metal element; z is the relative fraction of element MB with respect to the total amount of transition metals and comprises a real number between 0 and 1;

[0015] XA is a chalcogen element;

[0016] XB and Xcindependently are absent or chalcogen elements; yB is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1; yc is the relative fraction of element Xc with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;

[0017] 6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

[0018] In an embodiment, the present invention provides a material of Formula (IA) wherein:

[0019] MA is a transition metal element;

[0020] MB is absent or a transition metal element; z is the relative fraction of element MB with respect to the total amount of transition metals and comprises a real number between 0 and 1;

[0021] XA and XB are chalcogen elements; y is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;

[0022] 6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

[0023] In another embodiment, the present invention provides a material of Formula (IB): wherein:

[0024] MA is a transition metal element;

[0025] XA and XB are independently chalcogen elements; y is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1; 6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

[0026] In an embodiment, MA or MB are independently selected from W or Mo; XA, XB and Xc are independently selected from S, Se or Te; y is between 0 to 1; z is between 0 to 1; and 6 is between 0.01 to 0.5.

[0027] In another embodiment, the present invention provides a material characterized by a layered crystalline structure made up of sub-nanometer layers wherein each layer is between 0.25 to 1 nm in thickness.

[0028] In one embodiment, the material is ferromagnetic, ferroelectric and / or multiferroic. In another embodiment, the material is ferromagnetic or paramagnetic, and piezoelectric. In another embodiment, the material is ferroelectric and paramagnetic. In addition, the present invention also provide use of the said multiferroic material.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1. (a) Representative single crystals of W(Sei-yTey)2(i-5) (this case, for y=15% and d =21%) synthesized by chemical vapor transport. Scale bar is 1 mm long, (b) Crystal structure of a 2H-polytype of Te-doped WSe2 with space group P63 / mmc(#194).

[0031] FIG. 2. X-ray diffraction measurements for W(Sei-yTey)2(i-5) samples, (a) Powder X-ray diffraction data for a collection of undoped (y=0, 5=4%), and large Te-doping (y=15%, 5=21%) samples. Diffraction peaks are consistent with the 2H polytype, as compared with the reported peak positions of pure 2H-WSe2, shown by the vertical lines, (b, c) Close-up look to the (0 0 2) and (1 0 3) peaks, which reveal a shift to the left for the peaks of the Te-doped samples, indicative of an increase of about 2.5% in the c lattice parameter. XRD patterns for all tellurium-doped single crystals with (d) high, and (e) low levels of chalcogen vacancies.

[0032] FIG. 3. Raman spectroscopy measurements for W(Sei-yTey)2(i-5) single crystals with (a) high, and (b) low levels of chalcogen vacancies. Data shows a close-up look to the Raman shift around the characteristic E2gand Aigpeaks of the 2H phase, which shift to lower values with increasing doping. Vertical lines indicate the peak positions for the pure y=0 compound.

[0033] FIG. 4. Magnetic properties of single crystals of W(Sei-yTey)2(i-5). (a) Magnetization hysteresis loops at 300 K for samples with different y and 5 values. For all curves, the diamagnetic components were subtracted, and their values were normalized by the saturation magnetization, Ms. Inset in (a) shows the hysteresis loops for undoped (y = 0) WSe2(i-5) samples with different amounts of chalcogen vacancies, 5. (b) Magnetization curves for samples with small amounts of chalcogen vacancies, 5. Paramagnetic behavior is observed for these samples, independently of Te-doping. Coercive field as a function of (c) y, tellurium concentration, and (d) 5, chalcogen vacancies.

[0034] FIG. 5. Piezoelectric coefficient. DART-PFM piezoresponse amplitude as a function of the AC applied bias for W(Sei-yTey)2(i-5) samples with (a) y = 1.4%, 5 = 9%; (b) y = 7.6%, 5 = 38%; and (c) y = 15%, 5 = 21%. Scale bars of all three figures are 200 nm long, (d) Spatial average of the DART-PFM piezoresponse values from (a), (b) and (c) versus AC applied bias. The effective d33 piezoelectric coefficient is obtained from the slope of these curves. The curve for a sample of PPLN is included for comparison purposes. Error bar for each point represents the standard deviation of the amplitude within each image for each voltage.

[0035] FIG. 6. Switching Spectroscopy (SS) PFM hysteresis loops for W(Sei-yTey)2(i-5) single crystals, at 300 K under an inert atmosphere for (a) y=1.4%, 5=9%, (b) y=7.6%, 5=38%, and (c) y=15%, 5=21%. Insets to all phase curves show topography images indicating the exact locations at which the SS-PFM piezoresponse curves were performed, (d) Comparison of the SS-PFM amplitudes for the undoped and Te-doped samples.

[0036] FIG. 7. Current-voltage curves, IV, for single crystals of W(Sei-yTey)2(i-5) at 300 K (upper row) and 50 K (lower row). FIG. 8. Characterization of single crystals of W(Si-yTey)2(i-5) with three different concentrations: y=0, 5 <1% (this is, WS2(i-5)), y=0.1, 5 <30%, and y=0.4, 5 <50%. (a) X- ray diffraction measurements in powder configuration, with diffraction peaks consistent with the 2H-polytype. Shifts to lower angles for the peaks of the doped compounds, with respect to the original peak positions of the parent compound, WS2(i-5) (dashed vertical lines) can be observed for all the indexed peaks of the {001 } family of planes. This indicates a continuous increase in the c-lattice parameter with y-concentration. (b) Raman spectroscopy measurements for a representative sample of each composition. For the three compositions studied the observed Raman peaks are consistent with the 2H- polytype, with a slight shift in the position of the Aigpeak, (c) Paramagnetic contribution of the magnetization as a function of applied magnetic field for the y=0.4, 5 <50% sample.

[0037] FIG. 9. Characterization of single crystals of Mo(Sei-yTey)2(i-5) with two different concentrations: y=l, 5 <4% (this is, MoTe2(i-5)), and y=0.9, 5 =30%. (a) X-ray diffraction measurements in powder configuration, where a slight shift to lower angles can be observed for some of the peaks, particularly the most intense around 53 degrees, indicating a slight change in lattice parameter for this composition, (b) Raman spectroscopy measurements for a representative sample of each composition, indicating the presence of different polytypes for this compound.

[0038] DETAILED DESCRIPTION

[0039] Terms used in the following description have the meanings normally given to them in the technical field unless this description or the context clearly indicates otherwise. Where appropriate, terms used in the singular form shall also include the plural form. Unless otherwise indicated, by either contextual implications or customary practices, all parts and percentages in the present description are based on molar fractions. The term “approximately” means variations of ±5% of the defined value. The term “room temperature” as used herein refers temperature in the range of 4 °C to 50 °C; or between 5 °C to 35 °C, or between 10°C to 35°C, or between 15°C to 30°C. The term “multiferroic” as used herein refers to materials that simultaneously possess two or more ferroic orders, most commonly ferromagnetism and ferroelectricity, as well as materials that show one ferroic order and tendency to develop a second ferroic order through the presence of free dipole moments associated to this second ferroic order (for example, ferromagnetism, and large piezoelectricity due to formation of free electric dipoles). The materials of the present invention are multiferroic. The materials combine electrical (ferroelectric) and magnetic (ferromagnetic) properties and have strong correlation between these properties.

[0040] The term “chalcogen vacancies’" as used herein refers to unoccupied atomic positions in the crystallographic structure of the aforementioned TMDs which for the perfectly stoichiometric compound should be occupied by a chalcogen element. These vacancies are introduced during the synthesis process by a combination of chemical manipulation of the original stoichiometries of the precursors, chosen gas pressures during the sintering process, and chosen thermal profiles for the sintering and crystal growth.

[0041] In a first aspect, the present development corresponds to a room temperature multiferroic material of Formula (I): wherein:

[0042] MA is a transition metal element;

[0043] MB is absent or a transition metal element; z is the relative fraction of element MB with respect to the total amount of transition metals and comprises a real number between 0 and 1;

[0044] XA is a chalcogen element;

[0045] XB and Xc, independently are absent or chalcogen elements; yB is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1; yc is the relative fraction of element Xc with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;

[0046] 6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1. The room temperature multiferroic material of Formula (I) of the present invention, MA an p, are independently transition metal elements selected from W, Mo, Ta, Nb, V, Fe, Co, Mn, Ni, Cu, Ti, Hf, Zr, Pd, Pt, Au or Ag and wherein, particularly MA is W or Mo.

[0047] Furthermore, the chalcogen elements XA, XB and Xc are independently an element belonging to group 16 of the periodic table excluding oxygen. Particularly, XA, XB and Xc are independently selected from S, Se or Te and more particularly, XA is Se or S and XB is Te.

[0048] For the purposes of the invention, in the room temperature multiferroic material of Formula (I) yB is between 0 to 1; particularly between 0.01 to 0.99; or more particularly between 0.1 to 0.5; yc is between 0 to 1; particularly between 0.01 to 0.99; or more particularly between 0.1 to 0.5; z is between 0 to 1, particularly between 0.01 and 0.99; or more particularly between 0.1 and 0.8; and 6 is between 0.0 to 0.6; particularly between 0.01 and 0.5; or more particularly between 0.1 and 0.4.

[0049] In a second aspect, the present development corresponds to a room temperature multiferroic material of Formula (IA): wherein:

[0050] MA is a transition metal element;

[0051] MB is absent or a transition metal element; z is the relative fraction of element MB with respect to the total amount of transition metals and comprises a real number between 0 and 1;

[0052] XA and XB are chalcogen elements; y is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;

[0053] 6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

[0054] The room temperature multiferroic material of Formula (IA) of the present development, MA an p, are independently transition metal elements selected from W, Mo, Ta, Nb, V, Fe, Co, Mn, Ni, Cu, Ti, Hf, Zr, Pd, Pt, Au or Ag and wherein, particularly MA is W or Mo.

[0055] Moreover, the room temperature multiferroic material of Formula (IA) here development, the chalcogen elements XA and AB are independently an element belonging to group 16 of the periodic table excluding oxygen. Particularly, XA and AB are independently selected from S, Se or Te and more particularly, Ai is Se or S and AB is Te.

[0056] For the purposes of the invention, in the room temperature multiferroic material of Formula (IA) y is between 0 to 1; particularly between 0.01 to 0.99; or more particularly between 0.1 to 0.5; z is between 0 to 1, particularly between 0.01 and 0.99; or more particularly between 0.1 and 0.8; and 6 is between 0 to 0.6; particularly between 0.01 and 0.5; or more particularly between 0.1 and 0.4.

[0057] In a third aspect, the present development corresponds to a room temperature multiferroic material of Formula (IB): wherein:

[0058] MA is a transition metal element;

[0059] Ai and AB are independently chalcogen elements; y is the relative fraction of element AB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;

[0060] 6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

[0061] The room temperature multiferroic material of Formula (IB) of the present development, MA is a transition metal element selected from W, Mo, Ta, Nb, V, Fe, Co, Mn, Ni, Cu, Ti, Hf, Zr, Pd, Pt, Au or Ag and wherein, particularly MA is W or Mo.

[0062] Furthermore, the room temperature multiferroic material of Formula (IB) the chalcogen elements Ai and AB are independently an element belonging to group 16 of the periodic table excluding oxygen. Particularly, XA and XB are independently selected from S, Se or Te and more particularly, XA is Se or S and XB is Te.

[0063] For the purposes of the invention, in the room temperature multiferroic material of Formula (IB) y is between 0 to 1; particularly between 0.01 to 0.99; or more particularly between 0.1 to 0.4 and 6 is between 0 to 0.6; particularly between 0.01 to 0.5; or more particularly between 0.1 to 0.4.

[0064] In addition, the room temperature multiferroic material of any of the Formulas (I), (IA) and (IB) is characterized by a layered crystalline structure made up of sub-nanometer layers wherein each layer is between 0.25 nm to 1 nm in thickness, particularly between 0.5 nm and 0.9 nm; or more particularly between 0.6 and 0.8 nm. Moreover, the room temperature multiferroic material of Formulas (I), (IA) and (IB) are ferromagnetic, ferroelectric or multiferroic.

[0065] For the purposes of the present development, the room temperature multiferroic material of any of the Formulas (I), (IA) and (IB) further comprises traces of other transitions elements.

[0066] The term “traces” as used herein refers to concentration in the range of 10 % or less and the transitions elements are select from Ti, V, Cr, Mn, Fe, Ni, Co, Cu, Zn, Zr, Nb, Ta, Hf, Ir, Pd, Ag, Pt, or Au.

[0067] The synthesis of the multiferroic materials in the current invention can be carried out using any methodologies widely known in the field, as well as those that may emerge in the future, for example, by using solid state reaction (SSR), chemical vapor transport (CVT), chemical vapor deposition (CVD), or molten flux methods. These methodologies can be combined to achieve the material initially in its polycrystalline (powder) state through SSR, and can be followed by CVT to produce single crystals (distinct formations of the material). The primary distinction between the two methods lies in how the crystals are compacted. SSR utilizes mechanical pressure during compaction, which fosters cohesion among the material’s compounds and keeps the entire synthesis process in the solid phase. In contrast, CVT compacts the material in the gas phase with the aid of a carrier agent.

[0068] Thus, for instance in an embodiment of the development, powders with the desired stoichiometry are produced from elemental MA, B, XA, XB and Xc through a sintering process. For each batch, stoichiometric amounts of MA, MB, XA, XB and Xc are thoroughly ground and then cold pressed at between 3.448MPa (500PSI) to 10.342MPa (1500PSI); or between 5.516MPa (800 PSI) to 9.653 MPa (1400 psi) to form a pellet. The pellet is sealed in a quartz tube, leaving a small pressure of an inert gas such as argon or helium inside the quartz ampoule. The ampoule is then heated in a ramp from between 5°C to 35 °C (room temperature) to reach 400 °C to 1150 °C in between 2 h to 48 h; or between 450 °C to 850 °C in between 12 h to 24 h. This temperature is maintained for between 65 h to 150 h; or between 80 h to 130 h; or between 95 h to 115 h until the material is obtained in polycrystalline sintered form. The obtained sintered powder and the iodine as carrier agent were then encapsulated in vacuum in a long quartz tube and then placed in a two-zone tube furnace with between 700 ° to 1150 °C; or between 950 °C to 1100 °C; or between 1000 °C to 1050 °C in the hot zone and between 600 °C to 1100 °C; or between 850 °C to 950 °C; or between 880 °C to 930 °C in the cold growth zone for between 50 h to 240 h; or between 110 h to 160 h; or between 130 h to 150 h. The single crystals are collected from the growth zone of the tubes. The obtained crystals are properly stored in order to prevent sample oxidation and degradation.

[0069] However, it is clear for the skilled artisan that achieving specific relative molar fractions of z and and chalcogen atom vacancy fraction (5) in the single crystals do not necessarily exactly match those used during the stoichiometric mixing phase (which are said to be the nominal values). Instead, adjustments in the molar quantities of the stoichiometric mixture are made and in the synthesis conditions, with the anticipation that these adjustments will have a direct impact on the final fractions outcomes (z, y and 5). In addition, the chalcogen atom vacancy fraction (5) is strongly affected by the gas pressure during the sintering process, and we use this variable as a way to control the amount of chalcogen vacancies. Confirmation of the actual fraction values occurs only once the synthesis process is completed through X-Ray Fluorescence (XRF) analysis or other properly characterization techniques.

[0070] The present development will be presented in detail through the following examples, which are provided for illustrative purposes only and are not intended to limit its scope.

[0071] EXAMPLES

[0072] Example 1: Synthesis of W(Sei-yTey)2(i-g)

[0073] As mentioned previously, the preparation of the multiferroic materials of the present invention is carried out by any of the techniques known in the field. For example, Tellurium-doped tungsten diselenide single crystals, W(Sei-yTey)2(i-5) (with y = 0-18% and 5 = 1-38%), were grown by chemical vapor transport (CVT) with iodine as carrier agent. Particularly, powders with the desired stoichiometry were produced from elemental W, Se and Te through a sintering process. For each batch, stoichiometric amounts of tungsten (No. 357421, Sigma Aldrich), selenium (No. 36208, Alfa Aesar) and tellurium (No.266418, Sigma Aldrich) were thoroughly ground and then cold-pressed at 6.895 MPa (1000 psi) to form a pellet. The pellet was sealed in a quartz tube, leaving a small pressure of argon inside the quartz ampoule. The ampoule was then heated from room temperature to reach 500 °C in 24 h and this temperature was maintained during 72 h. The obtained sintered powder and the iodine were then encapsulated in vacuum in a long quartz tube and then placed in a two-zone tube furnace with 1010 °C in the hot zone and 900 °C in the cold growth zone for 144 h. The single crystals were collected from the growth zone of the tubes.

[0074] FIG. la shows a picture of representative single crystals of Te-doped WSe2, with typical lateral sizes around 1 mm, and a hexagonal morphology. A level of chalcogen vacancies is developed during the crystal growth process, and crystals with a general stoichiometry W(Sei-yTey)2(i-5) are produced. In this case, y represents the fraction of Se atoms that were replaced by Te atoms, and 5 is the fraction of chalcogen sites that are not occupied by neither a Se nor a Te. The doping values y and d of the W(Sei-yTey)2(i-5) single crystals were determined by triplication in X-ray fluorescence (XRF) using a ZSX Primus RIGAKU spectrometer. According to the stoichiometric formula, W(Sei-yTey)2(i-5), percentages of tellurium doping and chalcogen vacancies of the grown crystals are presented in Table 1. The “Nominal Composition” column presents the aimed Te-doping and chalcogen vacancies concentration, used to calculate the stoichiometric fractions of elemental precursors during the initial sintering process. However, the real compositions, determined through XRF, differ considerably from the nominal values, as shown in the last two columns (real concentrations of Te and vacancies).

[0075] Several Te-doping concentrations were obtained. In addition, two undoped compositions with a high-level of nominal vacancies (y=0%, 5=50%) were synthesized in order to discriminate the effects of Te-doping and chalcogen vacancies in the observed properties.

[0076] Table 1. Chemical composition of single crystals, determined through X-Ray Fluorescence Spectroscopy (XRF).

[0077] Example 2: Characterization of W(Sei-yTey)2(i-g)

[0078] Samples with different Te-doping values, plus undoped samples (y=0) with three different levels of chalcogen (Se) vacancies were characterized.

[0079] X-ray diffraction

[0080] Two different types of X-ray diffraction measurements (powder diffraction, presented in FIG. 2 and single crystal diffraction, presented in Table 2 reveal that the crystalline structure of all the pure and Te-doped compounds is the 2H-structure (space group P63 / mmc, # 194). This 2H-WSe2 (WSe2) crystal structure is shown in FIG lb and it is available at https: / / materials.springer.com / isp / crystallographic / docs / sd_0310430.

[0081] The X-ray powder diffraction patterns of W(Sei-yTey)2(i-5) single crystals for all compositions were obtained using an Empyrean PANalytical series 2 diffractometer with a CuKa radiation source ( = 1.5405980 A), operated at 40 mA, 45kV, and with a step size of 0.0262606° over a 29 range from 10° to 70°, in an Eulerian-Cradle geometry. This characterization was performed in a collection of single crystals of each batch, which were cut in tiny fragments and spread in the sample holder to emulate a powder diffraction experiment. This was done with the purpose of obtaining a statistically meaningful characterization of many crystals of each batch.

[0082] The most intense diffraction peaks in FIG. 2a come from the {0 0 1 } family of planes, given that the a-b plane of the crystals tends to align parallel to the sample holder. Close inspection to the {0 0 1 } diffraction peaks (FIG. lb) for the (0 0 2) peak) reveals a continuous shift to the left, and therefore, an increase of interplanar c-lattice parameter with increasing Te-doping as shown in FIG 2 (d-e).

[0083] Single crystal diffraction confirms the increase in c-lattice parameter and, in addition, reveals an increase of the in-plane a-lattice parameter with Te-doping. Table 2 shows a summary of a and c lattice constants found by X-ray diffraction.

[0084] Raman spectroscopy

[0085] Raman spectra for crystals of all the compositions studied were taken in a HORIBA Scientific XPLORA X1041210 Raman spectrometer. The excitation wavelength used for all measurements was 532 nm with a grating of 2400 lines / mm, in the range of 70 cm'1to 1000 cm'1. Raman spectra for all Te-doped compositions and an undoped one, shown a wide range of Raman shifts. A decrease in the Raman shift of the E2gand Aigmodes of the 2H-structure, with increasing Te-concentration is observed.

[0086] The 2H-polytype is also confirmed by Raman spectroscopy. Figure 3 (a-b) shows the Raman spectra for the different Te concentrations studied (y=0-18%), around the E2gand A igmodes for 2H-WSe2. A continuous decrease in the Raman shifts of both modes as tellurium concentration increases is observed, which also reveals a 2H-structure with continuously increasing a- and c-lattice parameters. Raman peaks associated with the ITd-polytype, which for pure WTe2 appear around 150 cm'1and 200 cm'1, are not present, implying that secondary phase domains with this structure are not detected.

[0087] Table 2. Lattice constants of the 2H-polytype structure, for all compositions obtained through powder x-ray diffraction and single crystal diffraction. For the powder X-ray diffraction, the values shown in the table are the average over different peaks of the same family.

[0088] Magnetic measurements

[0089] Isothermal magnetic hysteresis loops at 300K were measured in a LakeshoreTM 7400 Series vibrating-sample magnetometer (VSM). Magnetization measurements as a function of magnetic field at room temperature are shown in FIG. 4a, b. Magnetic response raw data reveals two components: a diamagnetic and a ferromagnetic contribution. Particularly, FIG. 4(a) shows the ferromagnetic component, obtained by subtracting the linear-diamagnetic contribution of each curve at 300K for y=0-15% tellurium-doped compositions, and 5 = 1-38%. The ferromagnetic component has a slight temperature dependence, particularly in the saturation magnetization. On the other hand, the diamagnetic component has a strong dependence with the level of oxidation of the samples. For instance, freshly synthesized samples showed no or minimum diamagnetic contribution, whereas the same samples after being stored in soft vacuum for a couple of days showed a marked diamagnetic contribution.

[0090] Samples with no Te doping (inset to FIG 4a) and low concentration of chalcogen vacancies (FIG 4b) show a paramagnetic behavior, with a saturating magnetization for fields above 150 Oe (fig 4a), and 2 kOe (fig. 4b), approximately. Te-free samples with an increased number of Se vacancies showed weak hysteresis loops with small coercive fields, indicating weak ferromagnetism. For all the Te-doped samples, the ferromagnetic hysteresis loops became more notorious, reaching a maximum coercive field of 700 Oe for the sample with y=7.6% and 5=38%. The coercive field as a function of chalcogen vacancies grows monotonically (Fig. 4d), in contrast to its behavior as a function of Te doping (Fig. 4c). This suggests that magnetism is driven by the amount of chalcogen vacancies. Nevertheless, the apparent strengthening of the ferromagnetism in the Te- doped samples, in contrast to the undoped samples, suggests some role of the Te atoms. This could be due to the fact that the presence of Te in the synthesis promotes the formation of chalcogen vacancies (Table 1).

[0091] Piezo- and ferro-electric response

[0092] The piezoelectric effect can be measured through piezoresponse force microscopy (PFM) technique, which is a variation of the atomic force microscopy (AFM) technique. The piezoresponse is acquired when an AC electrical voltage is applied using a conductive tip in contact with the sample surface. The resulting oscillatory deformation of the sample under the AC voltage is detected through the AFM cantilever deflection. Two kinds of PFM measurements were used: Contact resonance (Dual AC Resonance Tracking- DART) and Switching Spectroscopy - SS-PFM -, using a Cypher ES Environmental AFM operated at room temperature and in an inert N2 or Ar atmosphere.

[0093] In the DART-PFM mode the cantilever is operated with an AC voltage frequency near the contact resonance frequency, resulting in a driven harmonic oscillator, which enhances the piezoresponse signal, even if it is very small. Similarly, SS-PFM uses a sinusoidal voltage at contact resonance frequency overlapped with square voltages of smaller periods. In ferroelectric materials, it allows the determination of the electromechanical hysteresis loops and their switching parameters such as coercive and nucleation voltages.

[0094] To avoid misunderstandings in piezoresponse amplitude data, arbitrary units (a. u.) are used when piezoresponse amplitude is reported without treatment, whereas picometers (pm) are used when amplitude data is normalized by a single harmonic oscillator (SHO) model (Briscoe and Dunn, 2014. Nanostructured Piezoelectric Energy Harvesters. https: / / www.springerprofessional.de / en / nanostructured-piezoelectric-energy- harvesters / 2223256). Effective piezoelectric coefficient d

[0095] In order to check the calibration of our PFM experimental setup, and to gain confidence on the accuracy of the reported values for the piezoelectric coefficients of our single crystals, we performed DART-PFM and SS-PFM measurements in a commercially available Periodically Poled Lithium Niobate (PPLN) sample.

[0096] FIG. 5a-c shows the results of the DART-PFM measurements, in which the deformation amplitude is scanned in an area of about 3 pm><3 pm, and for different applied AC bias voltages, ranging from 0.5 V to 1.25 V. The amplitude values were normalized by the SHO model. The spatial average of deformation amplitude is plotted as a function of bias voltage in FIG. 5d. The slope of the linear regression to this curve corresponds to the piezoelectric coefficient, d . For our sample of PPLN, the experimental result du is 19.00±1.18 pm / V. Therefore, this demonstrate that the presented method is reliable for the determination of effective piezoelectric constants.

[0097] The spatially averaged piezoresponse as a function of AC bias, presented in FIG. 5d, is linear for all samples. The slope in each curve is equivalent to the effective piezoelectric constant d33 as previously mentioned. The two lowest Te-concentration samples exhibit comparable values for their effective d33, of the order of 4 pm V1. This value is similar to the piezoelectric constants previously reported for TMD monolayers, and higher than the du coefficient reported for a WSe2 monolayer. For the highest Te-doped sample, with y = 15% and 5 = 21%, d33 = (26 ± 4) pm V1. This remarkably high value is comparable to the d33 of PPLN — a device based on LiNbCL, a widely used piezoelectric material — and it is the highest reported or predicted d33 value among the TMDs in any configuration.

[0098] Piezoresponse force microscopy hysteresis loops

[0099] Switching Spectroscopy (SS) PFM hysteresis loops for different tellurium-doped compositions, are shown in Fig. 6a-c. The phase and amplitude of the SS-PFM piezoresponse were recorded as a function of the DC bias voltage at different random locations over the samples (insets to Fig. 6a-c). The y=1.4%, 5 = 9% Te-doped crystal (Fig. 6a) reveals a piezoelectric response without hysteresis. Interestingly, the amplitude tends to saturate for |VDC | > 1 V, suggesting a possible saturation of the electric dipole moment response. The y=7.6%, 5 = 38% and y=15%, 5 = 21% Te-doped samples show clear hysteresis loops both in the amplitude and phase of the SS-PFM piezo-response (Fig. 6b, c), with shapes equivalent to the ones shown in prototypical ferroelectric materials such as BaTiCh and PZT48. These loops indicate the presence of domains of electric polarization that can be aligned, with a coercive voltage of 0.2 V and 0.5 V for the y = 7.6% and 15% Te-doped bulk samples, respectively, and therefore reveal that these materials are ferroelectric. Provided that all ferroelectrics are piezoelectrics, both properties in this material should originate from the same mechanisms. In order to provide a comparison of the piezo / ferroelectric response for the different samples, Fig. 6d shows the amplitude response of SS-PFM for an undoped sample (y=0%, 5=4%), and the different Te-doped samples studied. Given that the undoped sample - which is not piezoelectric - was measured in the same conditions as the Te-doped samples, it can be used to identify the contribution of electrostatic effects during the measurement. It is worth noting that those contributions are negligible when compared to the experimental signal in our Te-doped compositions. Significantly, the amplitude of the SS-PFM signal increases enormously with increasing tellurium doping, which confirms the large enhancement of the piezoelectric coefficient for the highest Te-doping sample.

[0100] Transport properties

[0101] Another indication of ferroelectric properties in our single crystals can be found in the current vs voltage (IV) curves. FIG. 7 shows I-V measurements taken in samples from y=0% to 15%, at 300K and 50K.

[0102] Electrical properties of single crystals were measured by a four-probe technique for resistance vs temperature / field measurements, and two-probe technique for current vs voltage measurements, using a Keithley 2400 Source Meter. Gold pads were evaporated into the single crystals in order to reduce contact resistance and capacitive effects. Cryogenic temperatures were achieved using an Oxford Instruments liquid helium variable temperature insert cryostat equipped in an IntegraAC Recondensing Helium Magnet System.

[0103] Undoped WSe2(i-5) samples show an S-shape symmetrical curve, characteristic of a semiconductor, both at 300K and 50K, with no signs of anomalies. In contrast, all other tellurium-doped samples show IV curves that are highly asymmetrical with respect to the voltage polarity (diode-like behavior), and with clear hysteretic behavior (resistive switching-like behavior, and / or capacitive hysteresis-like behavior). This type of behavior seems to be enhanced at low temperature for all Te-doped samples. It is worth highlighting that the hysteretic and asymmetric behavior present at 300K for all Te-doped samples are commonly observed in prototypical ferroelectric materials such as BaFeCh and BaTiCh, and the origin of this multifunctional (i.e., diode-like, resistive switching and capacitive hysteresis effects) is an active area of research, with multifunctional applications in electronic devices.

[0104] In this case, a feasible mechanism for the existence of electric dipole moments in the 2H unit cell is the charge imbalance created by a chalcogen vacancy aligned with a chalcogen atom in the c-direction. The experimental results shows that no ferroelectric state is found in the pure WSe2(i-5), in which, although the charge imbalance mechanism due to vacancies is also present, it is possibly not strong enough to result in a ferroelectric state. Whereas ferroelectricity grows stronger with Te-doping, being well established for the largest Te-doped crystals.

[0105] For W(Sei-yTey)2(i-5), the role of chalcogen vacancies seems to be crucial for both ferromagnetic and ferroelectric states. Each of these ferroic orders in this material can exist independent from the other: WSe2(i-5) is only ferromagnetic and WTe2 is only ferroelectric. This, together with the fact that multiferroicity in W(Sei-yTey)2(i-5) is observed at room temperature, suggests that, although part of the origin of both ferroic orders can be common, this material is not a type-II multiferroic. In this class of multiferroics the mechanisms for both ferroic orders are highly intertwined through spinorbit coupling, and generally show cryogenic critical temperatures. Nevertheless, the strong influence of chalcogen vacancies on both ferroic orders in W(Sei-yTey)2(i-5) makes possible, not only the simultaneous presence of these states, but also their magnetoelectric coupling - a crucial ingredient in several envisaged applications of multiferroic materials. These ingredients, combined in an intrinsically 2D vdW layered material, and at room temperature, opens the door to a wide use of nano structured multiferroic devices.

[0106] Example 3: Synthesis and characterization of W(Si-xTex)2(i-6)

[0107] The single crystal synthesis of W(Si-yTey)2(i-5) follows the exact same procedures than described in example 1 for W(Sei-yTey)2(i-5). The only differences rely on the sintering and CVT temperature profiles. The most common sintering temperature for this solid solution is 850 °C, and the most common CVT temperatures are 1050 °C for the hot zone, and 950 °C for the growth / cold zone. Slight changes in the temperatures of the different process do not pose a significant impact on the results of the synthesis.

[0108] For the current solid solution single crystals with nominal compositions of 5=0 and y=0; 0.2; 0.5; and 0.7; and 5=30% and y=0.3; 0.5; and 0.7 have been produced. The results of the characterizations by X-ray diffraction, Raman spectroscopy and magnetization measurements for three representative compositions are shown in Fig. 8, with the real chemical composition values as measured through X-ray fluorescence are indicated in the plots.

[0109] X-ray diffraction and Raman spectroscopy measurements indicate a 2H-polytype structure for the three compositions presented, with y up to 0.4. XRD data (Fig. 8a) reveal a continuous increase in the c lattice parameter with increasing y. Raman spectroscopy (Fig. 8b) reveals an obvious shift to lower Raman shifts in the Aigmode, which is not as evident in the E2gpeak. More interestingly, there is an inverted trend in the relative intensities of the E2gand Aigpeaks, with the E2gpeak being the most intense for the y=0 sample (WS2(i-5)), but Aigbeing the most intense for both Te-doped samples. Magnetization measurements show a diamagnetic contribution, similar to results in W(Sei-yTey)2(i-5), and a clear paramagnetic contribution, as shown in Fig. 8c.

[0110] Example 4: Synthesis and characterization of Mo(Sei-yTey)2(i-g) The single crystal synthesis of Mo(Sei-yTey)2(i-5) follows the exact same procedures than described in example 1 for W(Sei-yTey)2(i-5). The only differences rely on the sintering and CVT temperature profiles, plus the incorporation of a rapid quench in iced water at the end of the CVT process. The most common sintering temperature for this solid solution is 900 °C, and the most common CVT temperatures are 1000 °C for the hot zone, and 950 °C for the growth / cold zone. When the tube in the CVT is at the end of the process and still at 1000 °C, the tube is rapidly removed from the furnace and put in a water / ice mixture in order to favor the IT’ and IT phases (over the 2H phase), which have shown tendency to be ferroelectric and topological in the undoped MoTe2 samples. Slight changes in the temperatures of the different process do not pose a significant impact on the results of the synthesis.

[0111] For the current solid solution single crystals with nominal compositions of 5=0 and y=0; 0.2; 0.5; 0.7; 0.9; and 1; as well as 5=30% and y=0.3; and 0.5 have been produced. The results of the characterizations by X-ray diffraction and Raman spectroscopy measurements for two representative compositions are shown in Fig. 9, with the real chemical composition values as measured through X-ray fluorescence are indicated in the plots.

[0112] Characterization of single crystals of Mo(Sei-yTey)2(i- 5) with two different concentrations closer to the MoTe2 side: y=l, 5 <4% (this is, MoTe2(i- 5)), and y=0.9, 5 =30% have been done through X-ray powder diffraction measurements and Raman spectroscopy, as shown in Fig. 9. XRD data reveals a slight shift to lower angles that can be observed for some of the peaks, particularly the most intense around 53 degrees, indicating a slight change in lattice parameters for this composition (Fig. 9a). Raman spectroscopy measurements for a representative sample of each composition (Fig. 9b) indicates the presence of different polytypes for this compound. The peaks labeled as Ai, Bgand Ag correspond to the IT’-polytype; the peak labeled as Aigto the IT-polytype; and the peak labeled as Eigto the 2H-polytype.

[0113] Example 4: Synthesis and characterization of other solid solutions The synthesis of single crystals of the following solid solutions has been carried with the concentrations as indicated below: Mo(Si-yTey)2(i- 5) with nominal values of 5=0 and y=0; 0.3; 0.5; 0.7; and 0.9; as well as 5=30% and y=0.3; 0.5; 0.7 and 0.9.

[0114] W(Si-yi.y2SeyiTey2)2(i- 5) with nominal values of 5=0, yi=0.4; and y2=0.4.

Claims

CLAIMS1. A material of Formula (I):wherein:MA is a transition metal element;MB is absent or a transition metal element; z is the relative fraction of element MB with respect to the total amount of transition metals and comprises a real number between 0 and 1;XA is a chalcogen element;XB and independently are absent or chalcogen elements; yB is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1; yc is the relative fraction of element Xc with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

2. The material according to Claim 1 of Formula (IA):wherein:MA is a transition metal element;MB is absent or transition metal elements; z is the relative fraction of element MB with respect to the total amount of transition metals and comprises a real number between 0 and 1;XA and XB are chalcogen elements; y is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

3. The material according to Claim 1 of Formula (IB)wherein:MA is a transition metal element;XA and XB are independently chalcogen elements; y is the relative fraction of element XB with respect to the total amount of chalcogen elements and comprises a real number between 0 and 1;6 is the fraction of chalcogen vacancies and comprises a real number between 0 and 1.

4. The material according to Claims 1 to 3, wherein MA and MB are independently transition metal elements selected from W, Mo, Ta, Nb, V, Fe, Co, Mn, Ni, Cu, Ti, Hf, Zr, Pd, Pt, Au or Ag.

5. The material according to Claims 1 to 3, wherein MA is W or Mo.

6. The material according to Claims 1 to 3, wherein XA, XB and Xc are independently an element belonging to group 16 of the periodic table excluding oxygen.

7. The material according to Claims 1 to 3, wherein XA, XB and Xc are independently selected from S, Se or Te.

8. The material according to Claims 1 to 3, wherein XA is Se or S.

9. The material according to Claims 1 to 3, wherein XB is Te.

10. The material according to Claims 1 to 3, wherein further comprises traces of other transition metal elements.

11. The material according to Claims 1 to 3, wherein: y is between 0 to 1; and z is between 0 to 1.

12. The material according to Claims 1 to 3, wherein 6 is between 0.01 to 0.5.

13. The material according to Claims 1 to 12, wherein the material is characterized by a layered crystalline structure made up of sub-nanometer layers wherein each layer is between 0.25 to 1 nm in thickness.

14. The material according to Claims 1 to 12, wherein the material is ferromagnetic.

15. The material according to Claims 1 to 12, wherein the material is ferroelectric.

16. The material according to Claims 1 to 12, wherein the material is multiferroic.

Images