Phase transformation in transition metal dichalcogenides

Devices including transition metal dichalcognides and methods of forming and operating such devices are disclosed. In one disclosed method, a layer of a transition metal dichalcogenide is provided, and a phase transformation of at least a region of the layer of the transition metal dichalcogenide is induced.

TECHNICAL FIELD

This disclosure generally relates to transition metal dichalcogenides (TMDs) and, more particularly, to phase transformation in TMDs.

BACKGROUND

The discovery of a mechanical exfoliation method for two-dimensional (2D) crystals was a watershed moment in the search for materials for the next generation of electronics and energy production. Among layered crystals amenable to isolation of atomically thin monolayers is a family of TMDs having the chemical formula MX2, where M is a transition metal, and X is S, Se, or Te. A semiconducting subset of TMDs includes those in which the transition metal M is Mo or W (both in Group 6), and has received attention in the pursuit of applications including ultrathin flexible electronics and spintronics. In an energy context, these materials hold promise as hydrogen evolution catalysts when certain features are exposed to a reacting environment.

It is against this background that a need arose to develop the techniques for inducing phase transformation in TMDs and related devices and methods described herein.

SUMMARY

In some embodiments, a method of operating an electronic or optoelectronic device includes: (1) providing a layer of a transition metal dichalcogenide; and (2) inducing a phase transformation of at least a region of the layer of the transition metal dichalcogenide.

In other embodiments, an electronic or optoelectronic device includes: (1) a layer of a transition metal dichalcogenide; and (2) a pair of electrodes connected to the layer of the transition metal dichalcogenide.

In further embodiments, an optical switching device includes: (1) a substrate; (2) a patterned metal layer disposed over the substrate; (3) a layer of a transition metal dichalcogenide disposed over the substrate and adjacent to the patterned metal layer; and (4) an electrical source connected to the layer of the transition metal dichalcogenide.

Other aspects and embodiments of this disclosure are also contemplated. The foregoing summary and the following detailed description are not meant to restrict this disclosure to any particular embodiment but are merely meant to describe some embodiments of this disclosure.

DETAILED DESCRIPTION

Monolayer TMDs are distinguished from graphene by the potential for multiple structural (e.g., crystalline) phases to exist, although the crystalline phases and properties of monolayers are largely yet to be elucidated. The presence of multiple crystalline phases in chemically exfoliated WS2, for example, can provide effective hydrogen production catalysis, indicating that the understanding and engineering of these phases can have immediate and important applications. In some embodiments of this disclosure, the energetic ordering of monolayer Mo- or W-dichalcogenide phases can be controlled through the application of strain or stress. The appropriate thermodynamic constraints are elucidated for monolayers, which can differ from bulk materials. Density-functional theory (DFT) and hybrid Hartree-Fock/DFT calculations are used to predict that the deformations to yield a phase transition range from about 0.5% to about 15% strain for this set of monolayers, falling within the range of elastic deformations afforded by the exceptional tensile strength of TMD monolayers. In other embodiments of this disclosure, phase transformation in TMD monolayers can be induced through the application of another stimulus or triggering mechanism in place of, or in combination with, mechanical stress or strain, such as through application of a voltage, an electrical current, excess or deficient charges, thermal stimulus, or optical stimulus (e.g., laser). For example, dynamic electrical control of conductivity in 2D materials is one of the most promising schemes for realizing energy-efficient electronic devices. Using DFT, it is determined that a structural semiconducting-to-metallic phase transition in TMD monolayers can be driven by electrical stimuli, including a change of charge density and bias voltage. The dynamical control of this semiconducting-to-metallic phase transition can be achieved by utilizing electronic devices like electrostatic gating employed in a field-effect transistor. These findings open up the possibility of manufacturing ultra-thin, flexible 2D phase change electronic devices with potential for higher energy efficiency than conventional electronic devices. More generally, the potential application space for the disclosed embodiments ranges from catalysis to information storage and nanoscale electronics, among others.

TMDs can be in the form of atomically thin monolayers having the chemical formula MX2, where M is at least one transition metal, and X is at least one of S, Se, and Te. In general, M can be selected from, for example, transition metals of Group 3 (e.g., Sc and Y), Group 4 (e.g., Ti, Zr, and Hf), Group 5 (e.g., V, Nb, and Ta), Group 6 (e.g., Cr, Mo, and W), Group 7 (e.g., Mn and Re), Group 8 (e.g., Fe, Ru, and Os), Group 9 (e.g., Co, Rh, and Ir), Group 10 (e.g., Ni, Pd, and Pt), Group 11 (e.g., Cu, Ag, and Au), and Group 12 (e.g., Zn, Cd, and Hg). Alloyed forms of TMDs are also encompassed by this disclosure, such as MmM′1-mX2, where M and M′ are different transition metals, and 0<m<1, MXxX′2-x, where X and X′ are different chalcogenides, and 0<x<2, and MmM′1-mXxX′2-x, where M and M′ are different transition metals, X and X′ are different chalcogenides, 0<m<1, and 0<x<2. Doped forms of TMDs are also encompassed by this disclosure, such as alkali metal-doped forms of TMDs. More generally, M can be any combination of one or more transition metals, X can be any combination of one or more of S, Se, and Te, and the chemical formula can be represented as MXy, where y is 2 or about 2. Of particular interest in some embodiments are TMDs in which the transition metal M is a Group 6 transition metal (e.g., Mo or W). In addition to TMDs, other two-dimensional materials (e.g., InSe, InTe, GaSe, and GaTe) can exhibit structural phase transitions, and are encompassed by this disclosure.

A special but often overlooked feature of these Group 6 TMD monolayers is that these materials can have more than one possible 2D crystal structure, which sets them apart from other 2D materials like graphene and BN. Intriguingly, one of these structural phases is semiconducting whereas the others are metallic. Co-existence of metallic and semiconducting regions on a monolithic MX2nanosheet can be used to form electronic devices, and metallic regions can promote catalytic activity leading to hydrogen evolution.

Despite the benefits of doing so, it has thus far proven to be challenging to form a stable metallic phase. Although a metastable metallic phase can be formed, it is uncertain that this phase would persist under realistic operating conditions. One would like to understand under what thermodynamic conditions metallic phases of TMDs are expected to be stable rather than just metastable, so that large areas can be synthesized using chemical growth techniques, like chemical vapor deposition. One would furthermore like to understand what TMD monolayers are nearest phase boundaries at ambient conditions and therefore most amenable to applications involving transformations between phases or mixed-phase regimes.

In this disclosure, DFT-based methods including thermal effects are used to determine the phase diagrams of freely-suspended TMD monolayers as a function of strain. It is determined that equibiaxial tensile strains of about 10-15% are involved to observe the 1T phase for many TMDs, but MoTe2can transform under considerably less tensile strain, less than about 1% under appropriate constraints. It is further determined that mixed-phase regimes can be thermodynamically stable under certain thermodynamic constraint conditions that are achievable in practice.

Under ambient conditions, Group 6 TMDs (except WTe2) can exist in a bulk crystal structure composed of monolayers where the X atoms are in trigonal prismatic coordination around the M atom. The atomic stacking sequence within a single XMX monolayer is βAβ. In keeping with typical nomenclature, this is referred to as the 2H phase, even though the prefix ‘2’ is irrelevant in monolayers because it refers to a bulk stacking mode. Group 6 MX2monolayers in the 2H crystal structure typically are semiconducting with band gaps between about 1 and about 2 eV. 2H-MX2materials are promising semiconductors for flexible electronic applications, among others. The 2H crystal structure gives rise to metallic edge states that are associated with electrocatalytic activity. The primitive unit cell of the 2H phase is hexagonal. For reasons of consistency between different phases, the calculations on 2H use a non-primitive rectangular unit cell whose axes align with zigzag- and armchair-directions of the structure. These special axes can be experimentally resolved using second harmonic generation and possibly also using the intrinsic piezoelectricity expected to exist in these materials.FIG. 1shows the 2H structure within a rectangular unit cell having lattice constants a and b.

When one of the 2H structure's X layers is shifted (for example, βAβ→βAγ), the X atoms are in octahedral coordination around the M atoms, and the crystal becomes metallic. This phase is referred to as 1T and is typically observed in Group 4 and Group 5 TMDs (e.g., TiS2and TaSe2). Its atomic structure is also shown inFIG. 1. The atomic vibrational normal modes (Γ-phonons) are calculated within the relaxed rectangular 1T unit cell of all six Group 6 TMDs. In all cases, one of the optical phonon modes has an imaginary vibrational frequency. This result indicates that the high-symmetry 1T structure is unstable (saddle point in 18-dimensional atomic potential energy surface), at least in the absence of external stabilizing influences.

Under certain conditions, the Group 6 TMDs have a stable metallic structure with octahedral-like M-X coordination. This lower-symmetry phase, which is referred to as 1T′, is a distorted version of the 1T structure. A rectangular (as well as primitive) 1T′ unit cell is displayed inFIG. 1. The 1T′ phase is observed in WTe2under ambient conditions, MoTe2at high temperature, and instances of chemically exfoliated and restacked MX2monolayers.

FIG. 2shows the calculated equilibrium (e.g., stress-free) relative energies of the phases (2H, 1T, and 1T′) of all six MX2monolayer materials. These values are calculated using DFT at the generalized gradient approximation (GGA) level of theory. The results are consistent with the experimental observation for bulk crystals that WTe2is the exception with 1T′ stable under ambient conditions. It is also observed from this data that the energy associated with the 1T to 1T′ relaxation is considerable: several tenths of an eV per MX2formula unit. Although phase transformation between 2H, 1T, and 1T′ crystal structures is considered in some embodiments, phase transformation between other crystal structures is also encompassed by this disclosure.

Thermodynamics asserts that a system will seek to minimize whichever thermodynamic potential is appropriate for the prevailing mechanical and thermal boundary conditions. An example of such a thermodynamic potential is the internal energy U. In the low-temperature regime, a system will seek to minimize U when it is constrained to a given shape or volume. Under these conditions, a MX2monolayer constrained to be described by a rectangular unit cell with dimensions a×b is expected to be in the lower-U phase for those values of a and b. Experimentally relevant phase diagrams of monolayers differ from those of bulk materials at high pressure in at least one important respect: The monolayer can be mechanically coupled to a substrate with friction, allowing the independent control of a and b lattice parameters. Another distinction is that tensile strains are achievable for monolayers, while compressive strains are achievable for bulk materials.

For the six Group 6 MX2monolayers, GGA DFT is used to calculate the energies U(a,b) of the monolayer crystal structures for a total of 49 pairs of (a,b)-values on a 7×7 grid of around the equilibrium lattice constants a0and b0. U is obtained after allowing the atoms to relax their positions within each unit cell. Intermediate values for U(a,b) of each phases are subsequently approximated using the Lagrange interpolation method:

Lagrange interpolation is chosen because it contains no physical assumptions about the shape of this energy surface over a large range of tensile and compressive strains. It also greatly facilitates the approximation of local derivatives without suffering from undesirable artifacts often manifested by high-order polynomial methods.

Using this approach, it is determined that the 2H and 1T′ U(a,b) energy surfaces intersect for sufficiently large strains.FIG. 3shows the contours that follow the intersection of the 2H and 1T′ energy in (a,b) space. The changes in a and b to change the relative energies of the 2H and 1T′ phases range from about 13% (MoS2) to about 3% (MoTe2). Because many bulk materials begin to dissipate strain energy through fracture or dislocations at strains on the order of 0.1% (roughly 10−5eV/atom), these threshold strains may at first appear to be prohibitively large. However, monolayer TMDs are exceedingly strong. For example, MoS2monolayers can reversibly withstand in-plane tensile stresses up to about 15 N/m, corresponding to about 10% of the material's in-plane Young's modulus. This corresponds to an elastic energy of order 0.1 eV/atom. The local derivatives of U(a,b) indicate that the stresses associated with the 2H-1T′ threshold strains are below about 15 N/m for all six Group 6 MX2compounds, indicating that a transition between 2H and 1T′ can be observable below but near the breaking threshold.

FromFIG. 3it is also seen that WTe2, which is usually in the 1T′ phase, can be induced into a 2H regime under compression, which is complementary to the other cases where the transformation would go from 2H to 1T′ through tension or expansion.

The results shown inFIG. 2andFIG. 3derive from the DFT-calculated potential energy U=Ucrystaland therefore omit the vibrational component of free energy. The vibrational component can be important when the energy difference between phases is less than about 0.1 eV, on the order of kBT.FIG. 2shows that the 2H-1T′ energy offset thus calculated for MoTe2is sufficiently small (about 43 meV) such that vibrational effects could play a role in the energetic ordering of phases. For a temperature-controlled evaluation, the Helmholtz free energy A(a,b,T) replaces U(a,b) as the relevant thermodynamic potential. This calculation is performed for MoTe2by treating the DFT-calculated vibrational normal modes as quantum mechanical harmonic oscillators. These effects in MoTe2are observed to have a considerable impact, even in the idealized T→0 K case because of contributions from the vibrational zero-point energy, which can be regarded as a manifestation of the wavelike nature of the atomic nuclei.

In the case of 2H- and 1T′-MoTe2, a temperature-dependent vibrational free-energy correction is calculated based on the frequency spectrum of the Γ-point phonons for each phase. These phonon frequencies are based on the second derivatives of the crystal energy with respect to the ionic positions within the unit cell. Second derivatives are calculated by applying perturbation theory to the relaxed ionic geometries for all 7×7 (a,b) grid points. The quasi-harmonic vibrational free-energy correction Avib=Uvib−TSvibis then added to the potential energy Ucrystal:

The Lagrange interpolation procedure is then carried out again at intermediate values of a and b. The top three curves inFIG. 4show that the vibrational correction Avibmoves the calculated (a,b)-space free-energy crossing in MoTe2to smaller strains. Indeed, a 0 K zero-point free-energy correction lowers the threshold strain by up to about 2% in some regions, and increasing the temperature from 0 K to 300 K produces yet another shift of up to about 2% in the same regions.

The results presented above are based on a generalized-gradient Perdew-Burke-Ernzerhof (PBE) functional for electronic exchange and correlation effects. Other exchange-correlation functionals can be used, such as a Hybrid PBE/Hartree-Fock approach known as the Heyd-Scuseria-Ernzerhof HSE06 functional. To explore how HSE06- and PBE-based predictions differ in a MoTe2context, the Ucrystalcomponent of Equation 2 is recalculated using the HSE06 functional on the 7×7 grid of PBE-relaxed geometries. (The phonon spectrum was not recalculated for this evaluation) The bottom three curves inFIG. 4show that use of the HSE06-calculated Ucrystalbrings the 2H-1T′ threshold strains even closer to the origin. At 300 K, the transition is predicted to be within about 2% strain of the equilibrium 2H lattice constants.

Consideration of the relevant thermodynamic constraints is provided here for the case of monolayers. The thermodynamic constraint of fixed lattice constants is not a typical one (different from constant volume or area), and is perhaps most applicable when metal atoms are constrained to their positions by substrate friction. At some fixed temperature T, the Helmholtz free energy A(a,b,T) is the thermodynamic potential whose minimization determines which crystal phase (or coexistence of phases) will exist in a crystal possessing a rectangular unit cell with dimensions a×b. However, this fixed-cell constraint need not apply in all conditions, and it is shown how to generalize the MoTe2results to other cases that are of practical relevance and allow the estimation of the width of any two-phase coexistence regimes in (a,b) space. Two-phase coexistence regions of bulk substances occur under constant volume conditions (e.g., liquid-vapor coexistence) and can be of technological relevance in monolayer materials. For example, a two-phase regime of WS2can provide enhanced H2production catalytic activity in WS2nanosheets.

In a “hydrostatic” isotropic-stress case, the natural thermodynamic potential governing the system is a Gibbs free energy Ghydroapplying to the “hydrostatic” isotropic-stress case where the surface tensions σxx=σyy=σ and σxy=0:
Ghydro(σ,T)=A−abσEquation 3

The previously independent variables (a,b,T) used in A=A(a,b,T) are now a function of (σ,T) through the definition of a 2D hydrostatic contour [a,b]=[a(σ,T),b(σ,T)]. This contour is determined directly from the interpolated F(a,b,T) surface and its local derivatives. This isotropic-tension condition is analogous to the isotropic constant-pressure case in three dimensions.

Another relevant case is that of a uniaxial load Fy=∂A/∂b along the crystal's y-axis, while the x-face is treated as a free surface (i.e., ∂A/∂a=0). Applying the appropriate Legendre transform to A for this case yields another Gibbs-like free energy Gythat acts as the governing thermodynamic potential.
Gy(Fy,T)=A−FybEquation 4

This situation would arise, for example, when a monolayer is suspended over a long linear trench, with its b-axis pointing along the direction that bridges the trench. A force Fycan be applied to the monolayer at the edges of the trench. In this case, the independent variables (σ, T) are mapped to a uniaxial contour [a,b]=[a(Fy,T),b(Fy,T)], derived from the interpolated F(a,b,T) surface and its local derivatives.

Both the hydrostatic and uniaxial contours are displayed inFIG. 5a(PBE-calculated Ucrystal) andFIG. 5b(HSE06-calculated Ucrystal). When increasing Fyor σ, at some point the Gibbs free energies of the 2H and 1T′ phases cross. When that happens, the result is a discontinuous jump in (a,b)-space from the 2H to the 1T′ contour, accompanied by a discontinuous jump in electrical, chemical, optical, magnetic, and other structural and non-structural properties. It is interesting to note that at this transition, any coexistence of both phases ranging from 100% 2H to 100% 1T′ is thermodynamically stable. The (a,b)-points at each end of the discontinuity mark the beginning and the end of a coexistence regime that is traversed when gradually increasing the width of the trench (e.g., b is directly controlled while the a-face is still free). This is true because the equality of the two phases Gy(Fy) implies that a free energy-minimizing two-phase common tangent exists in the A(a,b,T) ensemble, where a=a(b,T) such that ∂A/∂a=0. Similarly,FIG. 5shows that the hydrostatic stress case exhibits a mixed phase region that occurs under a constant area condition, in analog with constant volume for a bulk substance. Such a condition could be achieved by, for example, placing a monolayer over a hole in a substrate and clamping the edges fixed.

In the uniaxial-load case, the calculated strains εyy=b/b0−1 to enter the 2H end of this transition regime are shown inFIG. 6a. It is noted that the choice of exchange-correlation functional and the treatment of atomic vibrations have an impact on this number: εyyranges from about 0.49% (HSE06, 300 K) to about 2.38% (PBE, no vibrational free energy).FIG. 6bshows that the equibiaxial strain on the 2H end of the hydrostatic transition is similarly sensitive to temperature and electronic exchange-correlation functional. Finally, it is noted that the tensile load-specific transitions discussed here are closest to ambient conditions in MoTe2because of its small 2H-1T′ energy offset.

Additional thermodynamic ensembles can be applicable for monolayers. For example, when a monolayer is weakly bound to a substrate with friction, the atoms are allowed to move to some extent and restricted by contact with the surface. The thermodynamic potential of a TMD monolayer can be some intermediate case between the constant stress case (applicable to a frictionless substrate) and the fixed-(a,b) case where friction is large enough to inhibit any reorientation of the crystallographic unit cell.

The above discussion sets forth a mechanical route for thermodynamically driven phase transformations between metallic and semiconducting crystal structures in Group 6 TMDs. The results show that these mechanical transformations are realizable and particularly accessible in the case of MoTe2.

In general, mechanical stress or strain can be applied to an extent sufficient to induce a phase transformation between metallic and semiconducting phases, by varying (expanding/compressing) either, or both, the rectangular lattice constants a and b, namely along either, or both, the x- and y-directions shown inFIG. 1. The variation of the lattice constants a and b can be represented as percent engineering strains, normalized over the equilibrium lattice constants a0and b0in the absence of mechanical stress or strain, where the percent engineering strain of a can be, for example, a non-zero or any other value in the range of about −20% to about 20%, about −15% to about 15%, about −10% to about 10%, about −5% to about 5%, about −3% to about 3%, about −20% to about 0%, about 0% to about 20%, about −15% to about 0%, about 0% to about 15%, about −10% to about 0%, about 0% to about 10%, about −5% to about 0%, about 0% to about 5%, about −3% to about 0%, or about 0% to about 3%, and the percent engineering strain of b can be, for example, a non-zero or any other value in the range of about −20% to about 20%, about −15% to about 15%, about −10% to about 10%, about −5% to about 5%, about −3% to about 3%, about −20% to about 0%, about 0% to about 20%, about −15% to about 0%, about 0% to about 15%, about −10% to about 0%, about 0% to about 10%, about −5% to about 0%, about 0% to about 5%, about −3% to about 0%, or about 0% to about 3%. The magnitudes (in terms of absolute values) of the percent engineering strains of a and b can be the same or different, and the signs of the percent engineering strains of a and b can be the same or different. In some embodiments, the magnitudes and the signs of the percent engineering strains of a and b are optimized along a trajectory in (a,b)-space to induce a phase transformation between metallic and semiconducting phases, such as in terms of reducing or attaining a shortest path to cross an intersection contour of the metallic and semiconducting phases.

By changing a crystal structure from the 2H phase to the 1T′ phase (or vice versa), a TMD's electronic structure is modified (e.g., through a discontinuous jump in band gap or other electronic properties) to render it metallic (or semiconducting in the case of a transformation from the 1T′ phase to the 2H phase). In addition, other consequences of a structural transformation can be attained. For example, by changing the crystal structure, chemical properties of the TMD also can be modified (e.g., through a discontinuous jump in chemical properties), leading to, for example, different and desirable catalytic activity.

Another advantage of attaining a structural transformation relates to the stability of a resulting metallic phase. Metallic behavior sometimes can be observed when applying mechanical strain to a semiconducting phase (without undergoing a structural transformation), but that behavior can quickly disappear once the strain is removed. A metallic crystal structure, on the other hand, is more stable and can persist for years, even after strain is removed. This renders embodiments of this disclosure applicable to nonvolatile memory applications in which the contrast between semiconducting and metallic phases can be used to store information.

Mechanically-induced, solid-solid phase transformation can be achieved using flexible substrates, indirect substrate methods, gate pull-in methods, epitaxial growth, Atomic force microscopy (AFM) techniques, and other approaches.

For example, flexible substrates can be used in cases where phase transformations should occur in response to a mechanical manipulation of a device. A TMD monolayer can be deposited or otherwise applied on top of a larger, flexible substrate. Upon deformation of this substrate, the TMD experiences a strain commensurate with the substrate's deformation.

As another example, indirect substrate methods can be used to achieve a desired phase transformation. A TMD monolayer can be deposited or otherwise applied on a substrate, which is formed of a material that expands or contracts in response to a non-mechanical stimulus. Examples of such physical effects are: 1) piezoelectricity or piezomagnetism, 2) thermal expansion, 3) magnetostriction or electrostriction, and 4) shape memory alloys with strain hysteresis (e.g., nitinol). In the case of a piezoelectric substrate like lead zirconium titanate (PZT), a voltage can be applied across the substrate, leading to substrate deformation, which leads to TMD deformation, which in turn leads to a phase transformation within the TMD.

As another example, strain-driven phase transformations can be obtained in a TMD region that is not attached to a surface using a gate pull-in method, as illustrated for a memory cell inFIG. 7. The memory cell includes a TMD sheet1that is suspended over a trench with three electrodes2,3, and4(part a). By using an appropriately chosen pull-in voltage for the electrodes2,3, and4, an electrostatic attraction between the suspended TMD sheet1and the bottom electrode3pulls the sheet1into the trench (part b), causing tensile strain. After strain-driven transformation (part c), the pull-in voltage is released, and a metastable transformed region persists (part d). This metastable region can be removed by introducing a stimulus that reverses the phase transformation, such as a current between the left and right electrodes2and4(part e). The result of this operation has reset the TMD sheet1to its original phase (part f). The bit state of this memory cell can be measured using a conductivity or resistance difference between the semiconducting and metallic TMD sheet1.

As another example, a TMD sheet can be forced into specific lattice constants by chemically growing the sheet on top of a mismatching substrate, such as by epitaxial growth. This method yields a phase that is static, rather than dynamic that can be readily reversed. However, this method has the advantage that integrated circuits of metallic and insulating regions can be engineered by using a (lithographic) blueprint on the underlying substrate.

In addition to mechanically-induced phase transformations, phase transformations in TMDs can be induced through the application of another stimulus or triggering mechanism, such as through application of a voltage, an electrical current, or excess or deficient charges. For example, phase transformations can be triggered by electrical stimulus like electrostatic gating, utilizing conventional electronic devices. This electrically-triggered structural semiconducting-to-metallic phase transition in 2D materials involves moving electrons from or within whole materials, allowing controllable transition at small energy cost.

In some embodiments, DFT is used to determine the phase boundaries of two types of monolayer 2D Group 6 TMDs. One is monolayer MoTe2, for which the phase transformation involves the least energy, and the other is monolayer MoS2. The phase boundaries are calculated at constant charge and constant voltage, respectively. It is determined that a surface charge density of about −4×1013e/cm2or about 8×1013e/cm2is applied to observe the metallic phase in monolayer MoTe2and a larger value of about −34×1013e/cm2is applied in the monolayer MoS2case. For the constant voltage condition, the phase diagram of a monolayer MoTe2/MoS2placed on a substrate is calculated, with a bias voltage applied between them. It is determined that a bias voltage of about 0.5˜1 V is applied to induce the semiconducting-to-metallic phase transition in MoTe2, and a larger voltage of about 4˜5 V is applied to trigger the transition in MoS2. The phase boundary of the metallic-to-metallic phase transition in TaSe2is calculated and compared with experimental results, and reasonable agreement is observed.

FIG. 8shows the energy difference between 2H-MoTe2and 1T′-MoTe2versus excess charges in the unit cell (including 2MoTe2). When the unit cell is neutral, the 2H-phase has lower energy and is more thermally stable. However, when some excess electrons are added into the cell, namely negatively charging the cell, the energy difference decreases. When the unit cell is negatively charged with about 0.2 e−, the energy difference decreases to zero. When the cell is further charged, the 1T′-MoTe2phase has lower energy and becomes more thermally stable. Therefore, the phase transition point calculated using DFT is about 0.2 e−/unit cell, corresponding to a surface charge density of about 0.1466 c/m2. Negatively or positively charging other TMDs also can be used to attain charge-induced phase transformations, such as charging to a non-zero, surface charge density of up to, or at least, about ±1 c/m2, up to, or at least, about ±0.9 c/m2, up to, or at least, about ±0.8 c/m2, up to, or at least, about ±0.7 c/m2, up to, or at least, about ±0.6 c/m2, up to, or at least, about ±0.5 c/m2, up to, or at least, about ±0.4 c/m2, up to, or at least, about ±0.3 c/m2, up to, or at least, about ±0.2 c/m2, or up to, or at least, about ±0.1 c/m2. Also, by alloying different TMD materials, phase boundaries can be engineering to decrease the energy to induce the phase transformation.

It is thus determined that a transition between semiconducting 2H-TMDs and metallic 1T′-TMDs can be driven by excess electric charge (positive or negative) in a monolayer.FIG. 9presents an energy difference between these two phases varying with respect to a charge density in the monolayer. One example of excess electric charge in the monolayer is a charge transfer between the monolayer and a substrate, as shown in the inserted schematics ofFIG. 9. The substrate serves as an electron reservoir in this example, and it's assumed that a separation between the monolayer surface and the substrate surface is about 3 Å, a normal interface distance. InFIG. 9, the solid lines are stress free cases where both phases are structurally relaxed. This is applicable when the monolayer does not stick to the substrate. Regarding the dashed lines, the 2H phase is stress free, but the 1T′ phase is constrained to a computation cell of the same lattice constant as the relaxed 2H phase. This result is applicable when there is a strong interaction between the monolayer and substrate in which the monolayer sticks to the substrate and cannot relax freely.

FIG. 9ashows that semiconducting 2H-MoTe2has lower energy and is the equilibrium state when the monolayer is electrically neutral or minimally charged. For the stress free case (solid line), when the charge density is between about −3.95×1013e/cm2and about 8.42×1013e/cm2(e is the elementary electric charge), 2H-MoTe2is the thermodynamically stable phase. Outside this range, metallic 1T′-MoTe2will become the equilibrium state, and a transition from the semiconducting 2H phase to the metallic 1T′ phase will occur. For the strained 1T′ case (dashed line), a larger charge density is involved to drive the phase transition.FIG. 9bshows that, compared to the transition in monolayer MoTe2, the transition in monolayer MoS2is more difficult to induce and can be achieved by negative charging. If the negative charge density is larger than about 33.82×1013e/cm2, metallic 1T′-MoS2will have a lower energy; otherwise semiconducting 2H-MoS2will be energetically favorable. This is consistent with experimental observation that accumulating negative charge in monolayer MoS2can trigger trigonal prismatic to octahedral structural transformation. More generally, negatively or positively charging TMDs can be used to attain charge-induced phase transformations, such as charging to a non-zero, surface charge density of up to, or at least, about ±40σ, up to, or at least, about ±35σ, up to, or at least, about ±30σ, up to, or at least, about ±25σ, up to, or at least, about ±20σ, up to, or at least, about ±15σ, up to, or at least, about ±10σ, up to, or at least, about ±7.5σ, up to, or at least, about ±5σ, or up to, or at least, about ±1σ, where σ is 1013e/cm2.

FIG. 10presents a grand potential of a semiconducting 2H phase and a metallic 1T′ phase and the grand potential's changes with respect to an electron chemical potential. As inFIG. 9, here it's also assumed that a separation between a monolayer surface and an electron reservoir surface equals about 3 Å.FIG. 10ashows that, for a chemical potential between μt1=−6.19 V and μt2=−3.42 V, semiconducting 2H-MoTe2has a lower grand potential and is thermodynamically stable; outside this range, metallic 1T′-MoTe2has a minimized grand potential and becomes energetically favorable.FIG. 10bshows that, unlike monolayer MoTe2, monolayer MoS2has one transition chemical potential μt=2.01 V. When the electron chemical potential is lower than the transition potential, semiconducting 2H-MoS2has a lower grand potential and is energetically favorable; otherwise metallic 1T′-Mo S2has a minimized grand potential and becomes the equilibrium state. One example to test these results is to consider the surface layer of bulk 2H-MoTe2/MoS2as a monolayer placed on the bulk parent substrate. The electron chemical potential of the surface layer is about −Wbulk/e, where Wbulkis the work function of bulk 2H-MoTe2/MoS2(maroon vertical lines inFIG. 10).FIG. 10shows that the equilibrium state of the surface layer is a 2H phase, consistent with the observation that bulk 2H-MoTe2/MoS2is thermodynamically stable.

InFIG. 10, the top of the curve for semiconducting 2H-MoTe2and 2H-MoS2is relatively flat, the width of which should equal a band gap width. As a result, the change of the band gap would affect the top of the 2H phase curve. The grand potential values inFIG. 10are calculated using DFT under the generalized gradient approximation (GGA) for electronic exchange and correlation effects, using the Pedew-Burke-Ernzerhof (PBE) functional. PBE may underestimate the band gap; therefore one would expect the top flat region of the 2H phase to become longer at the right end, which corresponds to the conduction band. This change will shift μt2of MoTe2and μtof MoS2to the left for a short distance, but would not affect the existence of the transition chemical potentials.

A discussion of monolayer MoTe2/MoS2on their bulk parent crystals was included in the previous section. In certain applications, monolayer TMDs are placed on various substrates, which can affect an electron chemical potential of the monolayer and may change the equilibrium state of the monolayer. Also, it can be desirable to apply a gating signal to the monolayer TMDs to dynamically control the phase transition between the semiconducting 2H phase and the metallic 1T′ phase of Group 6 TMDs. If a bias voltage Vbiasis applied between the monolayer TMDs and a substrate with a work function Wsub, one can obtain an equation for Vbias, Wsuband the electron chemical potential of the monolayer μ:
Vbias=μ+Wsub/e(5)
where the monolayer is the reference for Vbias. For a given Vbiasand Wsub, one can obtain μ and then determine which phase is energetically favorable throughFIG. 10(which will change with the separation between the monolayer surface and the substrate surface s). In this manner, phase diagrams can be derived for monolayer TMDs with Vbiasand Wsubas the variables.

FIGS. 11aand 11bpresent phase diagrams for monolayer MoTe2and monolayer MoS2, respectively.FIG. 11ashows that, for a specific s, monolayer MoTe2has two phase boundaries. Between these two boundaries, semiconducting 2H-MoTe2has a minimized grand potential and is thermodynamically stable; outside this range, metallic 1T′-MoTe2becomes energetically favorable. The phase boundaries move with the value of s. A larger s will result in a wider region where the 2H phase is the equilibrium state. The separation s depends on the substrate, and a typical value for s is about 3 Å. Therefore, the phase boundaries would most likely lie between the boundaries for s=2.5 Å and the boundaries for s=3.5 Å (the shadowed regions). For a monolayer MoTe2placed on a substrate, the following considers the case where no bias voltage is applied (the horizontal grey line at Vbias=0). If the work function of the substrate is between about 3.59 eV and about 5.82 eV (s=2.5 Å), the equilibrium phase of monolayer MoTe2would be 2H; outside this range, 1T′-MoTe2is energetically favorable. Most common metal substrates have a work function between about 4.0 eV and about 5.5 eV, like copper (Cu) and gold (Au). Therefore it would be predicted that 2H-MoTe2is the more thermodynamically stable on these substrates. However, there are some metals with a smaller work function than about 3.0 eV, like lithium (Li).FIG. 11ashows that a Li substrate would favor 1T′-MoTe2rather than 2H phase.

FIG. 11bshows that, unlike monolayer MoTe2, monolayer MoS2has one phase boundary. Below this phase boundary, the semiconducting 2H-MoS2is energetically favorable; above the boundary, the metallic 1T′-MoS2has a minimized grand potential and becomes more thermodynamically stable. A bigger separation between the monolayer surface and the substrate surface will push the phase boundary toward the 1T′-equilibrium region.FIG. 11bindicates that, without a bias voltage applied, semiconducting 2H-MoS2is the equilibrium phase on any substrate.

FIGS. 11aand 11bindicate that, in addition to the substrate effect, electrostatic gating also has an influence on the thermodynamically stable phase of monolayer MoTe2and MoS2. To determine how the electrostatic gating would affect the equilibrium state of the monolayer on a specific substrate, one can draw a vertical line at the work function of the substrate inFIGS. 11aand 11b. Taking a monolayer MoTe2placed on a Cu substrate for example, which has a work function of about 4.65 eV, if no bias voltage is applied, the semiconducting 2H-MoTe2will be the equilibrium phase on top of the Cu substrate. A phase transition to metallic 1T′-MoTe2can be induced either by a positive bias voltage more positive than about 1.06 V (s=2.5 Å) or a negative bias voltage more negative than about −1.17 V (s=2.5 Å). A bigger separation s will increase these transition voltages.

For MoS2,FIG. 11bshows that a positive bias voltage can trigger the phase transition to metallic 1T′-MoS2. If a monolayer MoS2is placed on a Cu substrate and the separation s=2.5 Å, a positive voltage Vbias>5.22 V can drive the phase transition. A larger work function Wsuband a larger separation s will result in a larger voltage to trigger the transition.

More generally, applying a negative or positive bias voltage to TMDs can be used to attain voltage-induced phase transformations, such as applying a non-zero, bias voltage of up to, or at least, about ±10 V, up to, or at least, about ±9 V, up to, or at least, about ±8 V, up to, or at least, about ±7 V, up to, or at least, about ±6 V, up to, or at least, about ±5 V, up to, or at least, about ±4 V, up to, or at least, about ±3 V, up to, or at least, about ±2 V, or up to, or at least, about ±1 V.

It is noted that calculations of some embodiments of this disclosure are performed at 0 K, omitting the vibrational component of the free energy. Both inclusion of vibrational free energy and a temperature increase can shift the phase boundaries towards the 2H phase region and lower the energy to switch the phase. Therefore, the threshold charge density and bias voltage calculated in some embodiments can represent upper bounds.

Electrically induced structural phase transitions have been observed in other layered metal dichalcogenides. A solid-solid phase transition in a top layer of TaSe2can be induced by a STM tip. STM images of transition regions are obtained with a negative STM bias voltage from −1.2 V to −1.8V. To provide some insight into these experimental results and validate a total energy calculation approach for a charged monolayer of this disclosure, a phase diagram is plotted for monolayer TaSe2under a STM tip inFIG. 11c(See Examples for additional details of the calculations of TaSe2). Since a bias voltage is applied between the monolayer and the STM tip, the work function should be that of the STM tip rather than the substrate.FIG. 11cshows that monolayer TaSe2has one phase boundary. Above the boundary, 2H-TaSe2has a minimized grand potential and is thermodynamically stable; below the boundary, 1T-TaSe2becomes the equilibrium phase. The phase boundary depends on the separation between the monolayer TaSe2surface and the STM tip s. A larger separation will push the boundary towards the 1T′-TaSe2region. In the experimental results, it's most likely that the separation s is close to about 3.5 Å. Etched iridium (Ir) tips were used in the experiments, and Ir has a work function ranging from about 4.9 eV to about 5.3 eV. Based on the work function range of the Ir STM tip and the bias voltage applied, a shadowed box is drawn inFIG. 11cto display the experimental results of the phase transitions. The experimental box lies right below the phase boundary for s=3.5 Å, in which the phase diagram predicts that 1T-TaSe2is energetically favorable and a phase transition from 2H to 1T would be induced. This consistency helps to provide insight into the mechanism of the experimental results and also provide validation for the calculation method employed in this disclosure.

By way of summary, some embodiments of this disclosure are directed to a semiconducting-to-metallic phase transformation in monolayer TMDs driven by an electrical stimulus, like electrostatic gating. Some embodiments focus on the phase boundaries in monolayer MoTe2and MoS2, with the phase transformation in monolayer MoTe2being easier to induce. A bias voltage of about 0.5˜1 V can trigger the phase transformation in MoTe2, while a larger voltage is applied for the transition in MoS2. The threshold bias voltage also depends on a substrate on which the monolayer is placed. For MoTe2, the metallic 1T′ phase can be observed even without a bias voltage applied if the substrate has a work function smaller than about 3.5 V. Carefully choosing the substrate can greatly reduce the threshold bias voltage for the phase transformation, and therefore consume much less energy, indicating potential applications in electronics with high energy efficiency. To provide some insight into STM experimental results, a calculation is performed of the phase boundary of metallic-to-metallic phase transition in TaSe2, and reasonable agreement is observed between the calculation results and experimental data.

The understanding of phases and phase boundaries in TMD materials provides a path toward efficient and cost-effective hydrogen evolution catalysis materials. Furthermore, the existence of multiple phases in TMD monolayers has broad applications for electronic, microelectromechanical systems (MEMS), thermal, energy, and myriad other ultra-thin, flexible, energy-efficient, 2D devices which can benefit from the ultra-thin, flexible, transparent nature of the monolayers. For example, the applications of structural semiconducting-to-metallic phase transformation in phase change materials include phase change memories, field-effect transistors, infrared optical switching devices (e.g., as optical shutters, optical modulators, window coatings, or infrared optical nanoantennas), and energy storage devices, among other electronic or optoelectronic devices and applications.

For example, nonvolatile memories are important elements of integrated circuits due to their ability to maintain data even when not powered. Phase change materials are desirable for use in nonvolatile memories. Compared to conventional nonvolatile memory—flash memory, phase change memory has faster switching speed, longer lifetime, and better scalability, and is therefore a candidate to replace flash memory for nonvolatile data storage applications. A typical phase change memory includes at least one layer of a phase change material, which has at least two phases of different resistance. Phase transformation in conventional phase change materials is typically driven by temperature or heat, which is altered by a current pulse. As a result, one drawback of a conventional phase change memory is its sensitivity to temperature. Some embodiments of this disclosure are directed to a phase change memory device in which the phase transformation is electrically driven (or mechanically or non-mechanically driven), rather than temperature or heat, and therefore has reduced temperature sensitivity. Compared to a heat-driven phase transformation, an electrically-driven phase transformation in monolayer TMDs can consume less energy.

FIGS. 12, 13, and 14are schematic diagrams illustrating cross-sectional views of various embodiments of a phase change memory cell100,100′, or100″. As shown inFIGS. 12-14, the phase change memory cell100,100′, or100″ includes a memory element110formed of one or more phase change materials (e.g., one or more monolayer TMD materials) in which information is stored, two electrodes120and130through which a write voltage can be applied and a read voltage can be applied, and two conductive contacts140and150which can be used to program or read information stored in the memory element110. Advantageously, the phase change memory cell100,100′, or100″ is designed to have reduced temperature sensitivity and consume less energy.

The electrodes120and130can be connected to provide an electrical signal to the memory element110to store information. A voltage potential can be applied to the electrodes120and130through the conductive contacts140and150, as shown inFIGS. 12-14. Either of, or both, the electrodes120and130can be connected to an electrical source180(e.g., a voltage or current source) through the conductive contacts140and150. The electrodes120and130and the conductive contacts140and150can be formed of metals, metal alloys, or other suitable conductive materials.

The conductive contacts140and150can be address lines, such as column and row lines, which can be used to program or read information stored in the memory element110.

The memory element110, in which information is stored, can be formed of, or can include, one or more nonvolatile, phase change materials. These phase change materials can have electrical properties (e.g., resistance) that are changed by an electrical stimulus, such as the application of a voltage potential, an electrical current, or excess or deficient charges. Examples of suitable phase change materials include monolayer TMD materials. Monolayer TMD materials can exist in different crystalline phases, corresponding to different resistance, and their phase transformation can be driven by an applied voltage or excess or deficient charges.

Programming of the memory element110to alter its phase can be accomplished by applying a voltage potential to the electrodes120and130, thereby providing a voltage potential across the memory element110. The voltage potential can cause an electrical current to flow through the electrode120, the memory element110, and the electrode130.

In operation, either of, or both, a voltage and a current signal (e.g., in the form of a pulse) can be applied to the address lines140and150in order to program data into or read data from the memory element110. In some embodiments, a voltage potential can cause the memory element110to transform from a phase of higher resistance to another phase of lower resistance. A phase or state of higher resistance can represent a “reset” state (e.g., a logic “0” value), and a phase or state of lower resistance can represent a “set” state (e.g., a logic “1” value). The memory element110can be composed of a monolayer TMD. When a voltage potential greater than a threshold voltage is applied across the memory element110through the electrodes120and130, the monolayer TMD will transform between a phase of higher resistivity (e.g., 2H) and a phase of lower resistivity (e.g., 1T′), and therefore a “reset” state would be altered into a “set” state. In other embodiments, a phase or state of lower resistance can represent a “reset” state (e.g., a logic “0” value), and a phase or state of higher resistance can represent a “set” state (e.g., a logic “1” value). Both “reset” and “set” states can remain without energy (e.g., electrical, optical, mechanical, or a combination) applied to the memory element110.

The information or data stored in the memory element110can be read by measuring the resistance of the memory element110. In some embodiments, a read current can be provided to the memory element110using the electrodes120and130, and a resulting read voltage across the memory element110can be compared against a reference voltage using, for example, a sense amplifier (not shown). The read voltage can be proportional to the resistance of the memory element110. Therefore, a higher read voltage can indicate that the memory element110is in a higher resistance state (e.g., a “reset” state), and a lower read voltage can indicate that the memory element110is in a lower resistance state (e.g., a “set” state).

Insulating elements160and170can be included to electrically isolate the memory element110, the electrodes120and130, and the conductive contacts140and150from one another and from surrounding elements, as shown inFIGS. 13 and 14.

The phase change memory cell100,100′, or100″ shown inFIGS. 12-14can be positioned on a substrate (not shown). The substrate can be formed of a semiconductor, for example.

In some embodiments, the phase change memory cell100,100′, or100″ can include or can be connected to additional elements or structures. For example, the conductive contacts140and150can be connected to access devices or circuits. The access devices can include the electrical source180(e.g., a voltage or current source), and can be operated (e.g., turned on/off) to access the memory cell100in order to perform operations such as data programming. The access devices can be metal oxide semiconductor field-effect transistors (MOSFETs), but also can be devices other than MOSFETs. For example, the access devices can be a bipolar junction transistor (BJT) or a diode, among other types of access devices. In some embodiments, multiple phase change memory cells100,100′, or100″ can be included, such as in the form of an array.

In other embodiments, information can be stored and read according to optical properties (e.g., absorption, index of refraction, or both) of a memory element. For example, a bit state can be set using a laser pulse that changes a temperature or a charge distribution, and the bit state can be read from a resulting change in optical properties.

Other embodiments relate to energy storage applications. For example, an energy storage device can be charged by transforming from the 2H phase to the 1T′ phase, and the stored energy can be released by transforming back to the lower-energy 2H phase.

Other embodiments relate to field-effect transistors incorporating phase change materials. Field-effect transistors are the building blocks of various electronic devices, and the innovations in this field have fueled the exponential advances in the electronics industry. One aspect of the innovations is the shrinking of critical dimensions, which will eventually cease due to the fundamental physical limits of conventional materials. This challenge has motivated the search for alternatives to silicon. Some embodiments of this disclosure are directed to field-effect transistors using phase change materials, like TMDs. Monolayer TMDs can be used in transistors to overcome the scaling limit of conventional transistors. The semiconducting-to-metallic phase transformation can lead to a sharp increase in electrical conductivity; as a result, a sub-threshold swing can be much smaller than a limit of a conventional transistor.

FIG. 15illustrates a schematic of a field-effect transistor200using one or more phase change materials. A channel240is formed from one or more phase change materials, and is connected to three terminals, which are a source electrode210, a drain electrode220, and a gate electrode230. The phase change material channel240is disposed over a substrate250. The phase change material channel240can be formed of, or can include, one or more phase change materials, such as doped or undoped TMDs. For example, the phase change material channel240can be include a single layer, a few layers, or a bulk form of pristine MoTe2, pristine MoSe2, pristine MoS2, pristine WTe2, pristine WSe2, pristine WS2, alkali metal-doped MoTe2, alkali metal-doped MoSe2, alkali metal-doped MoS2, alkali metal-doped WTe2, alkali metal-doped WSe2, alkali metal-doped WS2, or a combination of two or more of the foregoing. The phase change material channel240also can include a single layer, a few layers, or a bulk form of alloys of doped or undoped TMDs. The substrate250can be a semiconductor substrate. For example, the substrate250can be a p-doped silicon substrate. The substrate250can be connected to additional elements or structures (not shown). For example, one or more electrical sources can be connected to the source, drain, and gate electrodes210,220, and230.

The gate electrode230can inject electrons or holes into the channel240. At a finite bias, there can be carrier accumulation in the channel240, and the device200is in the OFF state. At a large enough positive or negative bias, when the injected carrier density is large enough to induce semiconducting-to-metallic phase transformation, there is a sharp increase in the channel conductance and the drain current, and the device200is turned ON at this threshold voltage/transition voltage. The sub-threshold swing can be smaller due to the dramatically increased channel conductance.FIG. 16shows the sub-threshold behavior of a field-effect transistor using a phase change material and a conventional MOSFET.

Further embodiments of this disclosure relate to infrared optical switching devices incorporating phase change materials. Infrared optical switching applications include, for example, infrared optical shutters and infrared optical modulators for cameras and missile guidance systems, window coatings, and infrared antenna with tunable resonance. Certain phase change materials can exhibit a change in either of, or both, the transmittance of light and the refractive index at different wavelengths in infrared field when undergoing a phase transformation. This property allows the reversible manipulation of an optical response of various nanostructures in a nonvolatile manner. One type of phase change materials having this property is TMDs.

FIG. 17includes schematics illustrating cross-sectional views of infrared optical switching devices300,310, and320including phase change materials. As shown inFIG. 17, each infrared optical switching device300,310, or320includes at least one patterned metal layer330, at least one layer340of phase change materials, and a substrate350. The layer340of phase change materials can be formed of, or can include, one or more doped or undoped TMDs. For example, the phase change material layer340can include a single layer, a few layers, or a bulk form of pristine MoTe2, pristine MoSe2, pristine MoS2, pristine WTe2, pristine WSe2, pristine WS2, alkali metal-doped MoTe2, alkali metal-doped MoSe2, alkali metal-doped MoS2, alkali metal-doped WTe2, alkali metal-doped WSe2, alkali metal-doped WS2, or a combination of two or more of the foregoing. The phase change material layer340also can include a single layer, a few layers, or a bulk form of alloys of doped or undoped TMDs. The metal layer330includes nano-sized particles arranged in a desired array with suitable spacings, dimensions, and geometric shapes. A material of the nano-sized particles can be, for example, gold, aluminum, or other metal or metal alloy. The arrays of particles can have a square, rectangular, or spiral pattern, or can have another periodic, regular pattern, or an aperiodic, irregular pattern. The geometric shape of the metal particles can be spherical, spheroidal, rod-like, polygonal, or can irregular, and can be solid or hollow nanostructures. The metal particles can have at least one dimension in a range of up to about 200 nm, up to about 180 nm, up to about 160 nm, up to about 140 nm, up to about 120 nm, up to about 100 nm, up to about 80 nm, or up to about 60 nm, and down to about 10 nm, down to about 5 nm, or down to about 1 nm. The metal layer330is adjacent at least a portion of the phase change material layer340. For example, the metal layer330can be disposed over the phase change materials layer340, as shown inFIG. 17a, under the phase change material layer340, as shown inFIG. 17b, or between two phase change material layers340, as shown inFIG. 17c. InFIG. 17c, the top and bottom phase change material layers340can include the same phase change material or different phase change materials. The metal layer330and the one or more phase change material layers340are disposed over the substrate350. The substrate350can be a semiconductor substrate, such as a silicon substrate, or an insulator substrate, such as a glass substrate.

The optical switching devices300,310, and320can be reversibly switched between states of different optical transmittance to achieve a light blocking or filtering function, or between states of different refractive index to tune resonances of the metal layer330. An electrical source350can be connected to the phase change material layer340to apply a voltage bias (or other electrical stimulus) in order to switch the states of the phase change materials.

EXAMPLES

The following examples describe specific aspects of some embodiments of this disclosure to illustrate and provide a description for those of ordinary skill in the art. The examples should not be construed as limiting this disclosure, as the examples merely provide specific methodology useful in understanding and practicing some embodiments of this disclosure.

Phase Transformations Induced by Strain or Stress

Computational Details: DFT and hybrid DFT calculations were performed using the Projector-Augmented Wave (PAW) pseudopotential implementation of the Vienna Ab Initio Simulation Package (VASP), version 5.3. In the DFT calculations, electron exchange and correlation effects are described by the GGA functional of Perdew, Burke and Ernzerhof (PBE). Wave functions are expanded in a plane-wave basis set with a kinetic energy cutoff of 350 eV on an 18×18×1 Monkhorst-Pack k-point grid using a Gaussian smearing of 50 meV. The convergence thresholds were 0.5·10−6eV/MX2and 0.5·10−5eV/MX2for electronic and ionic relaxations, respectively. The linear-response phonon calculations use an electronic threshold of 0.5·10−8eV/MX2. The Hybrid DFT calculations employ the screened hybrid functional by Heyd, Scuseria and Ernzerhof (HSE06). The electronic convergence threshold used for HSE06 calculations is 0.5·10−5 eV/MX2. A total of 26 valence electrons per MX2are treated by HSE06 whereas the PBE calculations treat 18 valence electrons. The Lagrange interpolation used to calculate forces and stresses for the load-specific trajectories was carried out on a 6×6 grid instead of the usual 7×7 grid, leaving out the highest-strain points.

Computational unit cells of the 1T′ structure (shown inFIG. 1) possess reflection symmetry around the x=0 and x=a/2 axes. This symmetry seems to be present or very slightly broken in experimental data. Moreover, phonon analysis of this unit cell indicates that this reflection symmetry is stable under ambient conditions. 1T′ energies and intersection contours are calculated, such as inFIG. 3, using computational cells with the same numbers of atoms to allow description of potentially lower-symmetry 1T′ cells, but all except the highest-strain cases reverted to the x=0 and x=a/2 symmetric case when the atoms were allowed to relax within the strained unit cell. It is found that the 2H-1T′ intersection contours were virtually unaltered after allowing for broken symmetry.

Phase Transformations Induced by Electrostatic Gating

Electronic structure calculation: DFT calculations were performed using the projector augmented-wave pseudopotential implementation of the VASP, version 5.3.3. In the DFT calculations, electron exchange and correlation effects are described by the GGA functional of PBE. Wave functions are expanded in a plane-wave basis set with a kinetic energy cutoff of 350 eV on an 18×18×1 Monkhorst-Pack k-point grid using a Gaussian smearing of 50 meV. Calculations were performed at 0 K. The convergence thresholds were 0.5×10−8eV per MX2and 0.5×10−7eV per MX2for electronic and ionic relaxations, respectively.

Total energy of a charged monolayer: In an Ab initio simulation of a charged monolayer, a homogeneous background charge is automatically introduced in the vacuum space in order to compensate the excess charge so that the periodic computational cell remains electrically neutral. To obtain the total energy of a charged monolayer, corrections are performed to subtract the interaction between the electrons in the monolayer and the background charge. Also, to have the electron chemical potential fixed, a reference state is chosen, which is assigned zero energy and where the electron is placed after being removed from the monolayer. This reference state works as an electron reservoir and is placed in the middle of two periodic monolayers, namely the computational cell boundary. Therefore, half of the computational cell length in the vacuum direction equals the separation between the middle of the monolayer and the electron reservoir surface.

Ab initio simulation of a charged monolayer: In order to simulate a charged monolayer, a predetermined number of excess electrons (ne) are added to the system. neis negative if electrons are removed from the system. The electric charge of the monolayer is q=−nee, where e is the elementary electric charge. The homogeneous background charge is qbg=−q=nee, and the charge density is ρbg=qbg/Ω, where Ω is the computational cell volume.

The self-consistent energy obtained by ab initio calculations Escfis corrected for the interaction between the electrons in the monolayer and the background charge. Escfcan be described as:
Escf=Emo+Emo-bg+Ebg(6)
where Emois the energy of the monolayer without the background charge, Emo-bgis the interaction energy between the monolayer and background charge, and Ebgis the energy of the background charge without the monolayer. Decoupling the interaction between the monolayer and background charge essentially involves subtracting the second and third terms in equation (6), which can be represented as:

From equations (6) and (7), the monolayer energy can be obtained as:

In addition to the dependence on the number of excess electrons neshown in equation (8), the monolayer energy Emoalso depends on the computational cell length in the vacuum direction L; therefore Emo=Emo(ne, L). The second-order polynomial fitting of the ab initio result was performed using linear least squares regression method in order to obtain the analytical expression for Emo(ne, L).

Because electronic structure calculations were performed at zero temperature, the Helmholtz free energy F should be the same as total energy Emo. Therefore, the electron chemical potential μ can be represented as:

μ⁡(ne,L)=∂Emo⁡(ne,L)∂ne(9)
and the grand potential ΦGcan be represented as:
ΦG(ne,L)=Emo(ne,L)−μ(ne,L)ne(10)

Validation of the total energy calculation for a charged monolayer: In the previous section, the approach of determining the analytical expression for the monolayer energy Emo, the electron chemical potential μ, and the grand potential ΦGwas explained. To validate the approach, simulation was performed of a system of a monolayer MoTe2placed on a Li substrate. The energetics of 2H-MoTe2and 1T′-MoTe2will change with respect to the separation between the center of the monolayer and the substrate surface dMo-Li. In ab initio simulations, by fixing the Mo atoms in z direction (perpendicular to the monolayer plane), a plot can be made of the energy difference of 1T′-MoTe2placed on the Li substrate and 2H-MoTe2on the Li substrate (solid lines inFIG. 18).FIG. 18shows that, far away from the substrate, 2H-MoTe2has a minimized energy and is energetically favorable, and this is consistent with the observation that the equilibrium state of a free-standing MoTe2monolayer is the 2H phase. However, when the monolayer is pushed towards the Li substrate close enough, 1T′-MoTe2will have a lower energy and becomes thermodynamically stable. The transition separation dMo-Liat which 2H-MoTe2and 1T′-MoTe2have the same energy equals about 4.7 Å for a substrate of 4 Li layers (lower line) and about 4.6 Å for a substrate of 8 Li layers (upper line). Using the analytical expressions obtained for Emo, μ, and ΦG, a prediction can be made of the transition separation, given the condition that the electron chemical potential equals the negative of the work function of the substrate. The prediction result is about 4.10 Å using the work function of a substrate of 4 Li layers (right star), and about 4.16 Å using the work function of a substrate of 8 Li layers (left star). These prediction results are reasonably comparable to the simulation results. The difference may because the substrate of a few Li layers is not an ideal electron reservoir, indicated by the observation that increasing the number of Li layers would reduce the error between the simulation and prediction results.

Spin-orbit coupling effect: Band splitting due to spin-orbit coupling can play a role in the electronic calculations of TMDs. InFIG. 19, the energy differences between 2H-MoTe2and 1T′-MoTe2as a function of the charge density are compared between spin-orbit coupling-included calculations and spin-restricted calculations.FIG. 19shows that the inclusion of spin-orbit coupling effect would lower the threshold charge density for the phase transition.

Phase boundary of monolayer TaSe2at constant voltage: Similar to the phase boundary of monolayer MoTe2and MoS2at constant voltage plotted inFIG. 10, the phase boundary of monolayer TaSe2is plotted inFIG. 20.FIG. 20shows that increasing the separation between the monolayer and the electron reservoir would increase the negative threshold transition potential.

Vacuum electronic states: When excess electrons are assigned to the computation cell, a homogeneous positive background charge is automatically introduced in the vacuum space in order to compensate the excess charge. When the number of excess electrons is increased to some value and the vacuum separation in the direction perpendicular to the monolayer surface is greater than some threshold, vacuum electronic states of lower energy will form in the center of two periodic monolayers, namely the boundary of the computation cell. To avoid the formation of these vacuum electronic states, one can either reduce the number of excess electrons or decrease the vacuum separation.

Energy Efficiency of Electrically-Driven Phase Transformation

In this example, computation is performed of a minimum amount of energy to electrically drive a phase transformation in 2D MoTe2and MoS2. Next, the results are compared with an energy consumption of a phase transformation in Ge2Sb2Te5(GST), which is a phase change material used in phase change memories. The results show that the phase transformation in MoTe2involves about eight times less energy than the phase transformation in GST.

Based on calculation results using DFT, the electron chemical potential μ is plotted with respect to carrier density σ in monolayer MoTe2/MoS2for both semiconducting 2H phase and metallic 1T′ phase, as shown inFIG. 21. For MoTe2inFIG. 21a, DFT results predict two transition electron chemical potentials μt1=−3.42 V and μt2=−6.19 V (top and bottom horizontal dashed lines). Between these two lines, semiconducting 2H-MoTe2is thermodynamically more stable; outside these two transition values, metallic 1T′-MoTe2has a lower free energy and is the equilibrium phase. On these two transition lines, the monolayer is a mixed phase of semiconducting 2H structure and metallic 1T′ structure. In the mixed phase, a carrier density will change, while electron chemical potential will stay the same. Middle horizontal dashed line is the Fermi level Efof neutral 2H-MoTe2. Hence the area of the shadowed region Δσμ is the energy consumption to electrically drive a semiconductor-to-metal phase transition in monolayer MoTe2. For negatively charged-driven phase transition, the energy consumption is at least about E1=0.010 mJ/cm2; for positively charged-driven phase transition, the energy consumption is at least about E2=0.016 mJ/cm2. The same calculation is performed for MoS2, and the energy consumption is at least about E1=0.336 mJ/cm2.

The minimum amount of energy to melt and reset a GST bit is Emin=1.2 aJ/nm3. To compare the energy density calculated above with the energy consumption of GST, the calculated density is converted to three dimensions by dividing by the interlayer distance of MoTe2and MoS2, which is about 6.984 Å and about 6.147 Å, respectively. After conversion, the minimum energy to drive the phase transformation in MoTe2is about E1=0.143 aJ/nm3and about E2=0.229 aJ/nm3for negatively charged-driven and positively charged-driven, respectively (Table I). For MoS2, the minimum energy is about E1=4.811 aJ/nm3. As can be observed from Table I, the negatively charged-driven phase transformation in MoTe2involves energy about eight times smaller than the phase transformation in GST, indicating that MoTe2is a phase change material with higher energy efficiency than GST.

TABLE IMinimum amount of energy to drive phase transformation inMoTe2, MoS2and GST. Emin2Dis the energy density in twodimensions, and Emin3Dis the energy density in three dimensions.MaterialsEmin2D(mJ/cm2)Emin3D(aJ/nm3)MoTe2(neg.)0.0100.143MoTe2(pos.)0.0160.229MoS20.3364.811Ge2Sb2Te5(GST)—1.2

As used herein, the singular terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

As used herein, the terms “connect,” “connected,” “connecting,” and “connection” refer to an operational coupling or linking. Connected objects can be directly coupled to one another or can be indirectly coupled to one another, such as through another set of objects.

As used herein, the terms “substantially” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, the terms can refer to less than or equal to ±10%, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.