FIELD OF THE INVENTION

This invention relates to a multiferroic material and a method of preparing the same. In particular, the invention relates to a multiferroic material that possesses both ferroelectric and ferromagnetic properties and is stable at room temperature. The invention also relates to thin films formed from the multiferroic material and method of forming the thin films.

BACKGROUND

Ferroelectrics (FE) are electrically polarizable materials which possess spontaneous polarization below the Curie temperature (T _C E), and the polarization in ferroelectric materials is switchable with respect to an external electric field. Similarly, ferromagnets have induced magnetization below the Curie temperature (T _C E) and the magnetization in ferromagnetic materials is switchable with respect to an external magnetic field. A multiferroic property has both properties of ferroelectricity and ferromagnetism.

The field-driven switching of the ferroelectric and ferromagnetic properties forms the basis of ferroelectric random-access memory (FERAM) and magnetic random-access memory (MRAM) devices respectively. Both devices are non-volatile and have certain advantages over conventional random-access memory devices (RAMs). Materials that exhibit both a magnetization and a dielectric polarization in a single phase are referred to as multiferroic or magnetoelectric materials.

In a multiferroic material with strong magnetoelectric coupling, the polarization or magnetization will be switchable with respect to magnetic field or electric field. Therefore, the shortcomings in Ferroelectric Random-Access Memory (FERAM) and Magnetic Random- Access Memory devices (MRAMs) could be avoided by employing suitable multiferroic materials, such that low energy ferroelectric writing and non -destructive magnetic reading could be achieved.

At present there has been no single-phase bulk material reported that demonstrates long- range ordered switchable polarization and magnetization at room temperature.

The existence of room temperature ferroelectricity in metals has been a topic of intense research because the long-range dipolar electrostatic fields are usually screened by the conduction electrons of a metal. Despite significant progress made in the ferroelectric switching of semi-metal at low temperature, electrically switchable intrinsic electric polarization of metals at room temperature remains elusive both in bulk and two-dimensional (2D) scale. There are no known principles or guiding rule to how such a material can be fabricated or discovered.

Thus, there is a need for new and improved single-phase materials that exhibit a magnetoelectric effect over the typical operational temperature ranges of electronic devices (e.g. at room temperature). In addition, there is a need for materials that can easily be formed into multiferroic thin films using thin-film deposition techniques known in the art. Such thin films can be incorporated into a wide variety of electronic components, such as, for example, MRAM, FERAM components.

It is therefore desirable to provide a novel multiferroic material and a method of preparing the same that seeks to address at least one of the problems described hereinabove, or at least to provide an alternative.

SUMMARY OF INVENTION

In accordance with one aspect of the invention, a novel multiferroic material is provided. The multiferroic material comprises a compound having the following general Formula I:

MO _x A _y (I) wherein x is a value ranging from 1 to 3; y is a value ranging from 0.001 to 0.5;

MO represents a transition metal oxide material wherein M is a transition metal selected from the group consisting of Mo, W, Nb and V; and

A is a Group 16 element selected from the group consisting of Te, Se, Po and Lv.

In one preferred embodiment, M is Mo and A is Te, having a formula MoO _x Te _y , wherein x is a value ranging from 1 to 3 and y is a value ranging from 0.001 to 0.5.

In one embodiment, the material is multiferroic at a temperature between 100K and 400K. In another embodiment, the material is ferroelectric at a temperature between 100 and 373 K. In yet another embodiment, the material is ferromagnetic at a temperature between 2K and 400K.

In accordance with a second aspect of the invention, a thin film formed from the multiferroic material of the present invention is provided. In accordance with a third aspect of the invention, an electronic component comprising a thin film formed from the multiferroic material of the present invention is provided.

In accordance with a fourth aspect of the invention, a method for the preparation of the multiferroic material of the present invention is provided. The method comprises doping a transition metal oxide MO _x, wherein M is a transition metal selected from the group consisting of Mo, W, Nb and V and x is a value from 1 to 3, with one Group 16 element selected from the group consisting of Te, Se, Po and Lv.

In accordance with a fifth aspect of the invention, a method of forming a thin film from the multiferroic material of the present invention is provided. The method comprises depositing a thin film comprising the multiferroic material of the present invention on a substrate. In one embodiment, the method comprises growing MO _x A _y crystals on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will be apparent from a reading of the following detailed description and from the accompanying drawings.

Figure 1 shows the Vapor-trapped Two-step Chemical Vapor Deposition (VTCVD) growth of MoO _x Te _y crystals with well-controlled thickness and morphology. Figure 1(a) is a schematic illustration of the fabrication process of MoO _x Te _y crystals. Figures 1 (b) to 1 (g) show the optical images of MoO _x Te _y crystals with rhombus (b), near hexagon (c), and coalesced hexagon shape (d), and corresponding coalescence MoO _x Te _y films (e-g).

Figure 2 shows the structure and chemical composition of MoO _x Te _y crystals. Figures 2(a) and 2(b) show the bright field scanning transmission electron microscopy (BFTEM) images and corresponding selected-area electron diffraction (SAED) of the MoO _1.99 Te _0.01 crystals recorded along the [210] (a) and [110] (b) zone axis. Figures 2(c)-(e) show the Raman spectra (c); EDS (d), and XRD patterns (e) of MoO _x Te _y crystals with different elemental compositions.

Figure 3 shows the X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements of MoO _x Te _y crystals. Figures 3(a) to 3(c) show Mo 3d (a), Te 3d (b), and O 1s (c) spectra of MoO _x Te _y crystals. Figure 3(d) shows the UPS measurements of MoO _x Te _y crystals.

Figure 4 shows the piezoresponse force microscopy (PFM) measurements on MoO _x Te _y crystals. Figures 4(a) to 4(c) show the topological (a), amplitude (b), and phase images (c) of the MoO1 _. 99Te0.01 crystal with opposite polarization directions written using a PFM tip with ±10 V bias. Figures 4(d) and 4(e) show the Local PFM amplitude (d) and phase (e) hysteresis curves with a window bias of ±5 V.

Figure 5 shows the electronic properties of MoO _1.99 Te _0.01 crystals. Figures 5(a) and 5(b) shows the temperature-dependent resistivity (a) and MR verse magnetic field at different temperature (b) of MoO _{1 99} Teo. ₀₁ crystals with about 8 nm thickness. Figures 5(c) and 5(d) show the l-V characteristics at different maximum voltage ranging from 2 V to 6V (c) and different sweeping rate ranging from 0.1 V/s to 1 V/s (d) for about 8 nm (c) and about 200 nm (d) thick MoO _1.99 Te _0.01 crystals, respectively. Figures 5(e) and 5(f) show the evolution of the resistance as a function of the consecutive identical pulses of the 8 nm (e) and 200 nm (f) thick MoO1.99Te0.01 crystals.

Figure 6 illustrates the ferromagnetic properties of MoO _x Te _y . Figures 6(a) to 6(c) show the M-H curves measured at different temperature for MoO1.99Te0.01 (a), MoOi.gTeoi (b), and MoOi. ₅ Teo.5 (c) crystals. Figures 6(d) to 6(f) show the extracted M _s (d), M _r (e), and H _c (f) verse temperature for the corresponding type of crystals.

Figure 7 illustrates the device applications of MO _x A _y materials. Figures 7(a) to 7(c) show the device configuration for MRAM (a), FERAM (b), and Multiferroic memories (c).

DETAILED DESCRIPTION

The present invention provides a multiferroic material and a method of preparing the multiferroic material. The invention also provides a thin film formed from the multiferroic material and a method of the forming the thin film. The invention further relates to the use of the multiferroic material in electronic components and devices.

The term "multiferroic' ¹ as used herein refers to materials that simultaneously possess ferromagnetic and ferroelectric properties. The materials of the present invention are multiferroic. The materials combine electrical (ferroelectric) and magnetic (ferromagnetic) properties and have strong correlation between these properties. The materials exhibit a magnetoelectric effect.

In a first aspect, a multiferroic material comprising a compound having the following general formula I is provided:

MO _x A _y (I) wherein x is a value ranging from 1 to 3; y is a value ranging from 0.001 to 0.5;

MO represents a transition metal oxide material wherein M is a transition metal selected from the group consisting of molybdenum (Mo), tungsten (W), niobium (Nb) and vanadium (V); and

A is a Group 16 element selected from the group consisting of tellurium (Te), selenium (Se), polonium (Po) and livermorium (Lv).

In one preferred embodiment, the multiferroic material comprises a compound of formula MoO _x Te _y, wherein x is a value ranging from 1 to 3 and y is a value ranging from 0.001 to 0.5.

MoO _x Tey is a semimetallic material with high charge carrier concentration of more than 10 ²³ cm ³ and it is multiferroic at room temperature. Conventional mechanisms for ferroelectricity involve closed-shell d° or s ² cations, whereas ferromagnetic order requires open-shell d ⁿ configurations with unpaired electrons. This fundamental distinction has made it difficult to combine long-range order of the two dipoles in most materials but in the present invention, it has been surprisingly found that MoO _x Te _y and the other compounds with general formula I (MOxAy) can have both ferromagnetic and ferroelectric properties and stable at room temperature.

In one embodiment, the multiferroic material of the present invention is multiferroic at a temperature of 100K and 400K. In another embodiment, the material is ferroelectric at a temperature of 100K and 373K. In a further embodiment, the material is ferromagnetic at a temperature of 2K and 400K.

The quantity or concentration of a particular component of a given compound is specified as atomic percentage according to the stoichiometric formula. Using MoO _x Te _y as an example, if the ratio of Mo:0:Te is expressed as 1 :x:y, then x ranges from 1 to 3 and y ranges from 0.001 to 0.5.

In a second aspect of the present invention, a method for the preparation of a multiferroic material comprising the compound of the present invention is provided. The method comprises forming the multiferroic material comprising the compound of general formula I in a solid solution by doping a transition metal oxide MO _x , wherein M is a transition metal selected from the group consisting of Mo, W, Nb and V and x is a value from 1 to 3, with one Group 16 element selected from the group consisting of Te, Se, Po and Lv. "Solid solution" as used herein refers to a homogenous crystalline structure in which two or more elements or materials share a common crystal lattice. In a solid solution, the most abundant atomic form, or material, is referred to as the "solvent" and the less abundant atomic form, or material, as the "solute". Using the compound MoO _x Te _y as an example, M0O2 lattice acts as the solvent while Te is the solute.

In the present invention, the solid solution of MO _x (M = Mo, W, Nb, V) following a distorted rutile or monoclinic crystal structure is permeated with a solute element selected from the group consisting of Te, Se, Po and Lv.

It is very rare for material with ferroelectric or ferromagnetic properties to be stable at room temperature when the thickness of the material is ultrathin, for example, one with a thickness of less than 10 nm. Such ultrathin multiferroic material is very useful in ultrathin memory device applications.

In the present invention, the multiferroic material possesses both ferroelectric and ferromagnetic properties and the material is also stable at room temperature in that the material may maintain its performance for at least two months even when the material is fabricated into thin or ultrathin films. The thin and ultrathin films can be applied to electronic components. For most electronic components, the material will be in the form of thin films, typically thin films deposited on a suitable substrate.

In another aspect of the present invention, a thin film comprising the material of the present invention is provided. In particular, a thin film comprising a material consisting of a compound having the following general formula I is provided:

MO _X Ay (I) wherein x is a value ranging from 1 to 3; y is a value ranging from 0.001 to 0.5;

MO represents a transition metal oxide material wherein M is a transition metal selected from the group consisting of Mo, W, Nb and V; and

A is a Group 16 element selected from the group consisting of Te, Se, Po and Lv.

The term “thin film” as used herein is intended to mean a layer of a substance applied to a surface. Such a thin layer is one that has a thickness measured in dozens to hundreds of nanometers. In one embodiment, the film has a thickness ranging from 10 nm to 1 mhh. In another embodiment, the film has a thickness ranging from 50 nm to 80 nm, which may be described as "ultrathin film' ¹ . The method which will be described herein makes it feasible to deposit thin film and ultrathin film layers upon substrate means, such layers being of the order ranging from dozens to hundreds of nanometers, and shall herein be referred to as “thin film” in general.

Growth

The thin film consisting of the material of the present invention can be prepared by adjusting the growth conditions of the crystals forming the films, such as the growth time, and the hydrogen partial pressure. For example, a film with a thickness of about 50 nm can be grown under a gas mixture of argon and hydrogen (Ar/H2 of 200 sccm/20 seem) with a growth time of 20 min. The thin film can be fabricated to have dimensions appropriate for the particular application concerned. In one embodiment, the thin film has a thickness of 10 nm to 1 pm. In other embodiments, the thin film has a thickness of 50 nm to 80 nm. Regardless of the thickness, the thin film formed from the material of the present invention has ferroelectric and ferromagnetic properties.

In one embodiment, the growth of MO _x A _y crystals, including MoO _x Te _y (Molybdenum Oxy- telluride) crystals is based on using a vapor-trapped two-step chemical vapor deposition (VTCVD) method, but is not limited to this method only. The concept discussed herein of controlling the relative components of the material is also applicable to other growth methods known in the art, for example, Molecular Beam Epitaxy (MBE), Chemical Vapor Transport (CVT), Metal Organic Chemical Vapor deposition (MOCVD) and laser ablation deposition.

In an exemplary embodiment, the thin film comprising the material consisting of MoO _x Te _y has a thickness ranging from 10 nm to 1 pm. In one preferred embodiment, the thin film has a thickness of 15 nm to 25 nm and more preferably, about 20 nm and over 50 pm in size. The thin film is very stable under ambient conditions in that the thin film may maintain its performance for at least two months without any degradation. By means of in-plane electrical transport and piezoresponse force microscopy (PFM) measurements, the results show that the sheet resistance is only about 10 W/sq and the MoO _x Te _y film has switchable spontaneous polarization at room temperature. Further, vertical electronic transport and pulse measurements show resistive switching ratio of approximately 50 with a short switching time of 60ns for thin MoO _x Te _y crystals (with about 8 nm in thickness). This shows that the thin film has a potential for resistive random-access memory (RRAM) application.

Thicker crystals, with a thickness of about 190 nm to 210 nm, can also be grown. In memory switching applications, the resistive on/off ratio reaches up to 10 ⁴ and the resistance increase can be modulated by periodic of positive- and negative-bias pulses of 50 ps. Moreover, MoO _x Te _y also shows robust saturation magnetization of up to 117 emu/cm ³ at room temperature, 109 emu/cm ³ at 400K, and increase to 141 emu/cm ³ when the temperature decreases to 2K. These findings demonstrate that the combination of native metallicity, ferroelectricity, and ferromagnetism simultaneously is achievable and provides tantalizing potential for realizing multifunctional materials with unusual coexisting properties and nanoelectronics.

In one embodiment, the thin film is prepared by depositing the material of the present invention on a substrate. Essentially the deposition process relies in forming solid solution of MO _x (or M0O2 in the embodiment for MoO _x Te _y ) with one of the elements selected from the group consisting of Te, Se, Po and Lv which acts as the solute and permeating the MO _x (or M0O2) matrix at a concentration given by the formula MO _x A _y (or MoO _x Te _y ) wherein x = 1 to 3 and y = 0.001 to 0.5. Any suitable deposition technique known in the art may be used following this concept and composition window. Examples of suitable deposition techniques include pulsed layer deposition (PLD), atomic layer deposition (ALD), chemical vapour deposition (CVD), chemical vapour transport (CVT), molecular beam epitaxy (MBE), sputtering and physical vapour deposition (PVD).

In a preferred embodiment, the thin film is prepared using the vapor-trapped two-step chemical vapor deposition (VTCVD) method. The method comprises growing MO _x A _y crystals on a substrate in a two-zone reaction chamber wherein one end of the reaction chamber is sealed to form a vapour trapping tube for the growth of the MO _x A _y crystals on the substrate, thus depositing a thin film consisting of MO _x A _y on the substrate.

In one embodiment, the method comprises heating an M transition metal precursor selected from the group consisting of Mo, W, Nb and V in a zone upstream of the reaction chamber to a temperature of about 600°C to 800°C, preferably about 750°C. The temperature in the zone is maintained at the said temperature to allow M transition metal precursor to become a metal oxide MO3 and then to MC>3- _X . This is followed by introducing a vapour containing an element A selected from the group consisting of Te, Se, Po and Lv into the zone to allow the vapour containing the element to react with the MO3- _X species to grow the crystals and forming a thin film consisting of MO _x A _y compound on the substrate, wherein x = 1 to 3, and y = 0.001 to 0.5.

In one embodiment, the vapor containing the element A is introduced into the zone upstream of the reaction chamber by passing a carrier gas through a zone downstream of the reaction chamber where the element A is placed, whereby the element A is heated and maintained at a temperature between 500 to 600°C. The carrier gas carries the vapor of the element A as the carrier gas passes through the zone downstream where element A is placed and into the zone upstream for the vapor of element A to react with the MO3- _X species in the zone upstream of the reaction chamber. Any suitable type of carrier gases can be used including, but not limited to, argon (Ar), hydrogen (H ₂ ) or a mixture thereof, and nitrogen (N ₂ ) The growing of the crystals is performed under a controlled atmosphere so as to allow control over the crystalline structure and thickness of the thin film. The thickness of MO _x A _y with different morphologies is dependent on the ratio of Ar and H ₂ used in the method. A larger hydrogen partial pressure will increase the thickness of the MO _x A _y crystals as compared to a lower hydrogen partial pressure.

The thin film can be deposited onto any suitable types of substrate including, but not limited to, Si0 ₂ /Si, sapphire, quartz, highly-oriented pyrolytic graphite (HOPG), doped silicon, and mica substrates, etc.

Applications and uses

The multiferroic thin film of the present invention are useful for any applications in which materials having both ferroelectric and ferromagnetic properties can be used. The material of the present invention, and in particular the thin-film form thereof, are therefore particularly suited to incorporation into electronic components and devices.

Hence, in another aspect of the present invention, an electronic component comprising the material consisting of MO _x A _y as defined herein, or a thin film of the material defined herein is provided. In one preferred embodiment, the material used is MoO _x Te _y , where x = 1 to 3 and y = 0.001 to 0.5.

In various embodiments, the electronic component is selected from the group consisting of a memory device, a tunnel junction, a magnetic field sensor, a transmitter, a receiver, a transmitter-receiver module, a phase array system and a resonator. In one embodiment, the electronic component is a memory device. In another embodiment, the memory device is selected from MRAM, FERAM or Multiferroic memories. The multiferroic thin film can replace the magnetic electrode of the MRAM or replace the ferroelectric film of the FERAM, while with the rest of the device structures remain unchanged. Examples of these embodiments are shown in Figure 7. Figure 7(a) shows an exemplary configuration of the multiferroic material in a MRAM device. The MRAM device has at least two multiferroic materials (MO _x A _y ), with each of the multiferroic materials sandwiched between an electrode and an insulator. Figure 7(b) shows an exemplary configuration of the multiferroic material in a FERAM device. The FERAM device has a multiferroic material (MO _x A _y ) sandwiched between an insulator and an electrode. Figure 7(c) shows an exemplary configuration of a multiferroic memory element. The multiferroic memory element has a multiferroic material (MO _x A _y ) sandwiched between two electrodes. Suitable techniques known in the art can be used to prepare such electronic components.

In a further aspect, the present invention provides an electronic device comprising an electronic component as defined herein. In various embodiments, the electronic device is selected from the group consisting of a tunable microwave device (e.g. attenuator, band bass filter or phase shifter).

In yet another aspect, the present invention provides use of MO _x A _y compound as defined herein as a multiferroic material, and use of the multiferroic material in fabricating thin films for use in electronic components and devices.

The following examples illustrate various embodiments of this invention. One skilled in the art will recognize that the examples set out below are not an exhaustive list of the embodiments of this invention.

EXAMPLES

Example 1

1 . Vapor-trapped Two-step Chemical Vapor Deposition (VTCVD) process

The inventors have developed a VTCVD process for growing crystals of the material of the present invention. In this example, we illustrate the use of VTCVD process in growing MoO _x Te _y crystals. In this process, argon (Ar) and hydrogen (H ₂ ) were used to carry molybdenum (Mo) precursors (ammonium molybdate, AHM) mixed with sodium cholate, and tellurium (Te) powder into a reaction chamber, to grow high-quality 2D large-scale MoO _x Te _y crystals on silicon dioxide/silicon (SiCVSi) substrate, as depicted in Figure 1 (a). Unlike the widely used setup in which substrates are put face-down above the molybdenum(VI) oxide (M0O3) source, the setup in this example employed a two-zone furnace and a smaller quartz tube with one end sealed (vapor trapping tube) for the growth. Prior to the growth process, SiCVSi substrate was first sonicated in acetone and isopropanol for 10 min each, followed by 20 min of oxygen plasma to remove the impurities absorbed on the surface of the substrate. The vapor trapping tube containing 60 mg of Te powder was located upstream of Zone I (“Z1”), and the open end extends to the center of Zone II (“Zll”).

The Mo precursors (20 mg) were placed on a quartz substrate which was introduced into the open end of the vapour trapping tube. The SiCVSi substrate was placed at the downstream side just next to the Mo precursors. The CVD growth process was carried out at ambient pressure, and the temperatures of Te powder (in Zone I) and Mo precursor (in Zone II) during the growth were kept at about 500°C to 600°C and about 600°C to 800°C, preferably 750°C, respectively. The key step of growing MoO _x Te _y crystals using VTCVD process is that the temperature in Zone II should be held at about 600°C to 800°C, preferably 750°C for 5 min (step I) to allow two events to take place: (i) the decomposition of ammonium molybdate (AHM) to molybdenum(VI) oxide (M0O3), and then (ii) to M0O3- _X under Ar/H atmosphere. Following, in step II, the Te vapour was introduced to react with M0O3-C species to form rhombus MoO _x Te _y (R-MoOTe), as shown in Figure 1 (b).

It is to note that a vapor trapping tube is used in the present case as the Te powder should not be carried into the reaction zone under Ar/H ₂ atmosphere before the Mo precursors reached 750°C, otherwise Te would react with M0O ₃ and only MoTe ₂ would be grown.

When the temperature in Zone I was increased from 500°C to 550°C, the morphology of MoO _x Te _y crystals changed from rhombus to near hexagon (N-MoOTe, see Figure 1(c)). With a further increase of the temperature to 600°C, the morphology eventually transformed into coalesced hexagon (C-MoOTe, see Figure 1 (d)). In addition, the thickness of MoO _x Te _y with different morphologies is strongly dependent on the ratio of Ar and Fl ₂ . Large hydrogen partial pressure (for example, Ar/H ₂ of 200 sccm/20 seem) would significantly increase the thickness of the MoO _x Te _y crystals (Figures 1(e) to 1 (g)) as compared to low hydrogen partial pressure (for example, Ar/H ₂ of 200 sccm/5 seem, Figures 1 (b) to 1(d)).

2. Characterization

In order to characterize the structure and chemical composition of MoO _x Te _y crystals with different morphologies, bright field scanning transmission electron microscopy (BF-STEM), selected-area electron diffraction (SAED), Raman, energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoelectron spectroscopy (UPS) were performed as shown in Figure 2 and Figure 3. Atomic resolution BF-STEM images for R-MoOTe crystals were recorded along the [210] and [110] crystallographic direction (see Figures 2(a) and (b)), which roughly matches the crystal structure with monoclinic molybdenum dioxide (m-Mo0 ₂ ). Importantly, the SAED patterns, as shown in the inset of Figures 2(a) and (b), show additional superspots (marked by a circle), which is due to the ordered Te substitution.

Raman spectra further confirmed the different chemical composition of the MoO _x T e _y crystals. For MoO _1.99 Te _0.01 crystals, the main modes of 128, 205, 230, 348, 362, 498, and 569 enr ¹ were observed, which is slightly red shifted compared to the lattice vibration modes of m- MO0 ₂ due to Te substitution of oxygen (MoO _{1 99} Te ₀₀₁ Figure 2(c)). With increasing Te substitution, a peak at 163 cnr ¹ arises, which represents the A _g mode of MoTe2 (MoOi _.g Teo _.i in Figure 2(c)). Higher Te concentration would bring more A _g mode (112 cm ¹ ) and stronger intensity of MoTe ₂ (163 cnr ¹ ), as shown in the topmost curve in Figure 2(c) (MoOi sTeo.s)· EDS measurements indicate that the R-MoOTe crystal is composed of Mo, O and Te, with an atomic ratio of Mo and Te at about 1 : 0.01 (MoO1.99Te0.01, the bottommost curve in Figure 2(d)). The N- and C-MoOTe crystals were also characterized, which show the ratio of Mo and Te at about 1 :0.1 (MoOi.gTeo.i,) and about 1 :0.5 (MoOi.sTeo.s,), respectively (see Figure 2(d)).

X ray diffraction (XRD) technique is used to track the evolution in the crystal structure as a function of elemental composition of Mo, O and Te in MoO _x Te _y . As shown in Figure 2(e), the MoOi.sTeo.s crystals exhibit strong diffraction peaks at around 13°, 26°, 39°, 53°, which are near to the (002), (004), (006) and (008) crystal planes of the 1T’ MoTe ₂ phase, respectively. The maximum intensity found in 26° indicates a high crystallinity with the (004) plane being preferentially oriented. For the MoOi _.g Teo _.i crystals, similar XRD patterns were observed with the appearance of the peak of 33°, which indicates the (101) crystal plane of M0O2. When the Te concentration is reduced further (MoO1.99Te0.01), as observed in the bottommost curve in Figure 2(e), the (002) crystal plane for the 1T-MoTe ₂ phase gradually disappear and new strong peak at 18° appeared but shifted toward a lower 2Q degree, compared to the (-101) crystal plane of M0O2, due to the expansion of the M0O2 lattice induced by Te.

To examine the bonding states and work function in MoO _x Te _y , XPS and UPS measurements were performed. As shown in Figure 3, for the XPS spectra of the Mo 3d core levels, all the crystals exhibit emission peaks from Mo 3d _5/2 and 3d _3/2 , which could be fitted with two spin- orbit doublets for each state, corresponding to Mo ⁴⁺ and Mo ⁶⁺ oxidation states, respectively. According to the peak area of the Mo ⁴⁺ (229.1 and 232.3 eV) and Mo ⁵⁺ (231 .8 and 234.9 eV) states, the concentration of Mo ⁴⁺ is much higher than that of Mo ⁵⁺ , which clearly confirms that the molybdenum ion in the sample is basically tetravalent. From the Te 3d core levels, we can see a large portion of Te is bonded with Mo (573.1 eV and 583.8 eV), whereas Te- O bond only constitutes a small component. The appearance of the 01s peak at approximately 532.8 eV further corroborates the presence of the 0-Mo bond with very small amount of O-Te at approximately 530.5 eV, indicating that the introduction of Te mostly substitute the O site instead of Mo site. Furthermore, a series of XPS spectra on MoO _x Te _y demonstrated that as Te content increases, the O content decrease, such that the atomic ratio of Mo, O and Te is 1 :1.98:0.02, 1 :1.89:0.11 , and 1 :1.46:0.54 for rhombus, near hexagon, and coalesced hexagon shaped crystal, respectively, which are roughly consistent with the value obtained by EDS. UPS measurements were performed using a UHV system equipped with a He discharge lamp and the spectra were recorded by using unfiltered He I (21.22 eV) excitation and the excitation source with the sample biased at 3V to observe the low-energy secondary cut-off (Figure 3). All the three compositions of MoO _x Te _y share the same secondary electron cut-off (13.76 eV) and fermi lever (0 eV). Considering the biased voltage of 3V, the calculated work function would be 4.46 eV for all the MoO _x Te _y crystals.

3. Piezo Force Microscopy (PFM) study of MoO _x Te _y crystals.

The investigation of polarization switching and measurements of hysteresis curves was carried out by using dual AC resonance tracking using PFM imaging and conducting PFM in spectroscopy mode (PFS). Figures 4(a) to 4(c) show the topography, amplitude and phase image after poling processes with biases of 10V and -10V for MoO _1.99 Te _0.01 crystals. It can be seen that the phase in the area poled with a -10V and 10V tip bias shows a clear trend of the domain switching with the amplitude image showing an expanding response as well. The positive and negative tip bias can switch the polarization up and down, respectively. The polarization switching behavior is further verified by PFS techniques. Fully saturated polarization switching is confirmed by a phase angle difference of 180° in the phase hysteresis loops and the butterfly-like shape observed in the amplitude loops with a window bias of ±5 V (Figures 4d and e). These results clearly show that room temperature ferroelectric polarization switching can be achieved in MoO _1.99 Te _0.01 crystals.

4. In-plane and out-of-plane electronic properties of MoO _1.99 Te _0.01 crystals.

Figure 5(a) shows the temperature dependence of in-plane electrical resistivity of approximately 8 nm thick MoO1 99Te0.01, measured to 2K. MoO1.99Te001 crystal shows metallic behavior as the resistivity increases linearly with temperature, which varies from 4.37 10 ⁹ W-m at 2K to 4.51 10 ⁸ W-rn at 300K. The magnetoresistance (MR) for fields up to 10 T was also measured as shown in Figure 4(b). In the perpendicular field (Hi) at 2K, the fractional MR change, AR/R ₀ = (RH - Ro)/Ro, is about 38% between 10 and 0 T, which would significantly decrease with increasing the temperature (Figure 5(b)). The dominant carrier in this crystal is electron, and the mobility is 137 cm ² /Vs with the carrier density of 1.09x10 ²³ cnr ³ at 2K, while at room temperature, the mobility is decreased to about 1 cm ² /Vs with the carrier density of 22.3 x 10 ²³ cnr ³ .

In addition, the out-of-plane electronic properties were also measured, in which a bias voltage was applied between the gold (Au) top electrode and graphene bottom electrode. Figure 5(c) shows the polarization and resistive switching properties at room temperature for the MoO1 99Te001 crystal with approximately 8 nm thick. At a low maximum bias voltage of 2V, a very weak l-V hysteresis is observed. The l-V hysteresis loop starts to develop at 3V, and the loop grows with further expand as the maximum voltage increases to 6V. It is worth noting that the resistance is very low, with only 6V forcing the current to the compliant value of 0.1 A, but still have an on/off ratio of about 50. For the thicker crystal with about 200 nm in thickness, the resistance is much higher, and the switching voltage is much increased with a resistive on/off ratio of up to 10 ⁴ (Figure 5(d)). The hysteretic response is also consistently observed at different sweep rates. As the field sweep rate increases, the coercive field increases, which is remarkably different with traditional ferroelectric materials.

To further investigate the switching speed to toggle between the high resistive state (FIRS) and a low resistive state (LRS), pulse measurements on both thin and thick MoO1 _. 99Te0.01 crystals were performed. Consecutive trains of positive and negative pulses were applied on the devices with the fixed number of pulses to 50. The potentiation and depression voltage for thin crystal is 5V and -5V with the reading voltage of 1 V, while for thick crystal is 10V and -10V with the reading voltage of 2V, respectively. Interestingly, the resistance for thin crystal immediately transitioned from FIRS (approximately 270 W) to LRS (approximately 8 W) when the applied bias reached -5V, without the electroforming process, and the resistive switching time is within 60 ns (Figure 5(e)). More cycles with the nearly constant HRS and LRS confirmed a stable and reproducible memristive switching of this device, which provide a great potential for resistive random-access memory (RRAM) application. Differently, for the thick crystal, the resistance increases exponentially and decrease suddenly with the repetition of positive- and negative-bias pulses of 50 ps (Figure 5(f)), which can be used to mimic excitatory and inhibitory synapses in organisms. Also, the resistance gradually increases with the number of positive pulses, which indicates the cumulative effect existing in this device.

5. Ferromagnetic properties of MoO _x T e _y crystals.

The magnetization characteristics of the MoO _x Te _y crystals with different Te concentration through superconducting quantum interference device (SQUID) measurements were investigated. Figures 6(a) to 6(c) shows the magnetization vs magnetic field (M-H) curves of MoO _x Te _y crystals. All the MoO _x Te _y crystals show the enhanced magnetization and ferromagnetic hysteresis loops when decreasing the temperature. For instance, at 2K, the MoO1 _. 99Te001 shows the largest saturation magnetization (M _s ) of 141 emu/cm ³ , remanent magnetization (M _r ) of 55 emu/cm ³ , and coercive magnetic field (H _c ) of 191 Oe. With the increase of the temperature, the magnetization is slightly decreased and the sample still maintains its ferromagnetic hysteresis loop even at 400K with M _s of 108 emu/cm ³ , M _r of 31.6 emu/cm ³ , and H _c of 105 Oe, of which the same principle can be also found in MoOi _.g Teoi (Figure 6(b)) and MoOi _.5 Teo.5 (Figure 6(c)) crystals. Interestingly, although the M _s and M _r of MoO1.99Te001 crystals are always higher than that of MoOi.gTeo.i and MoOi. ₅ Teo.5 crystals (Figures 6(d) and 6(e)), the H _c remains lower in the whole temperature range, which is mainly because magnetic crystals are more closely interwoven with each other when the concentration of Te increases. The Curie temperatures of the MoO _x Te _y crystals are 650K, 51 OK, and 460K for MoO1.99Te0.01, MoO1.9Teo.i, and MoOi ₅ Teo.5, respectively.

The above is a description of the subject matter the inventors regard as the invention and is believed that those skilled in the art can and will design alternative embodiments that include of this invention as set forth in the following claims.