Transition metal-based material phase engineering

Abstract

A method of converting a semi-conducting phase of a layered transition metal-based material to a metallic phase. The method comprises treating the semi-conducting phase with light of photon energy that is greater than a bandgap of the semi-conducting phase in the presence of a reducing agent. Also disclosed are methods of phase patterning a layered transition metal-based material and layered transition metal-based materials obtained or obtainable by the methods disclosed. The layered transition metal-based material can be a transition metal dichalcogenide.

Description

[0001] TRANSITION METAL-BASED MATERIAL PHASE ENGINEERING

[0002] This application claims priority from GB 2312904.2 filed 23 August 2023, the contents and elements of which are herein incorporated by reference for all purposes.

[0003] The project leading to this application has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 758826).

[0004] Field of the Invention

[0005] The present invention relates to a method of converting semi-conducting phases of layered transition metal-based materials to conducting phases and methods of patterning comprising such conversion methods. The present invention also relates to products obtained or obtainable by such methods of conversion and patterning.

[0006] Background

[0007] In this disclosure, indications such as [1] identify reference disclosures, the full bibliographic details of which are set out at the end of the description.

[0008] Transition metal dichalcogenides (TMDs) display a wide range of electronic properties based on their crystallographic structure [1 , 2]. Of particular interest is the phase transition from a semi-conducting phase to a conducting metallic phase (the trigonal prismatic 1H, 2H phase to the octahedral 1T / 1T’ phase in M0S2, for example) [1-3]. This phase transition has been exploited to enhance field effect transistor device performances by lowering the contact resistance [3-5], increase the hydrogen evolution reactivity [6-9], and unlock a wide range of chemical exfoliations and functionalization down to single layer TMDs [10-13]. More recently, it has also been used to advance flexible rectifying antennas

[0014] and lithiumsulfur battery technology

[0015] .

[0009] Chemical lithiation using n-butyllithium (n-BuLi), which was first proposed in 1975

[0016] , remains the most popular route to achieve the semi-conducting to metallic phase transition. Here, the semiconducting TMD is immersed in n-BuLi, which yields the metallic phase after several days. Mechanistically, n-BuLi first donates an electron to the semi-conducting TMD, which enables Li-ion intercalation into the van der Waals (vdW) gap to complete the phase transition [1 , 16]. This charge-transfer limited reaction mechanism is further supported by electrochemical studies, which identified the electrochemical landscape of the overall reaction as a sequential process of lithium adsorption through initial charge transfer, followed by the phase transition and irreversible phase conversion at lower potentials [17-19].

[0010] Traditional methods of chemical lithiation using n-BuLi are slow, taking place over a few days, which limits their practical utility. In addition, n-BuLi is pyrophoric, which limits the range of safe reaction conditions available. For example, adding external energy via heat or sonication can be dangerous. More reactive reagents, such as t-BuLi have been used, but the same problems arise

[0020] . Attempts to accelerate phase change via chemical intercalation methods which depart from the traditional route include molten metal intercalation [21 , 22]; electrochemical intercalation [23, 24]; methods employing redox potentialmatching of reagents

[0025] ; and the addition of more chemical steps, such as the use of pre-expansion steps

[0026] . Attempts to accelerate phase change in these ways can be dangerous, can require complex experimental setups and can see unsatisfactory results. Specifically, electrochemical intercalation requires specialized cells resembling batteries, which adds additional complexity to the method and molten metal intercalation requires high temperatures up to 600 °C and specialized furnaces.

[0011] Spatially-controlled chemical lithiation with n-BuLi is frequently used in the few- or monolayer limit to spatially inscribe metallic phases, which enhances the performance of field-effect transistors by lowering the contact resistance between 2D TMD and a metal contact [3]. However, because of the slow and uniform phase transition reaction across the whole material, inscribing the different phase within a 2D atomic layer always requires pre-patterning steps, and takes days to complete which limits the productivity and increases costs.

[0012] The present invention has been devised in light of the above considerations.

[0013] Summary of the Invention

[0014] The present inventors have realised that it is advantageous to be able to carry out a semi-conducting to metallic phase change in a layered transition metal-based material and to carry out such a phase change quickly, easily and safely. Furthermore it is advantageous to be able to carry out such a phase change spatio-selectively, such that phase patterning of the layered transition metal-based material can be carried out.

[0015] In a first aspect, the present invention provides a method of converting a semi-conducting phase of a layered transition metal-based material to a metallic phase of the layered transition metal-based material, comprising treating the semi-conducting phase with light of photon energy that is greater than a bandgap of the semi-conducting phase in the presence of a reducing agent.

[0016] Here, a semi-conducting phase is any phase (i.e. which may be some part or region of the material) which features a band gap between a (filled) valence band and an (un-filled) conduction band. A metallic phase is any phase (i.e. which may be some part or region of the material) which features a partially filled band in its band structure (for example, the valence and conduction bands overlap), and as such exhibits a higher conductivity than the semi-conducting phase. It may be that the metallic phase is conducting or is quasi-metallic, such terms may be used interchangeably with “metallic” to describe the target metallic phase of the invention.

[0017] The semi-conducting and metallic phase can also be characterized structurally, and the conversion is associated with a structural change in the material, for example a change in metal coordination geometry. Such a change in structure / geometry alters the electronic structure and properties of the material. Here, “layered” may be used to describe a material which is characterized by comprising layers, such layers comprise weaker inter-layer forces than intra-layer forces. For example, the intra-layer forces (bonds) may be described as being covalent, ionic or a combination thereof, while the inter-layer forces may be described as electrostatic or Van der Waals (equivalently, London or dispersion) forces (bonds). Herein, “layered” also includes “monolayered” materials (i.e. the layered transition metal-based material can be a monolayer).

[0018] The semi-conducting phase is treated (i.e. exposed to or illuminated by) light of a photon energy that is greater than a bandgap of the semi-conducting phase in the presence of a reducing agent. As such, the actual means of illumination is not particularly limited, so long as light of appropriate photon energy is included in the treatment. The light that the material is exposed to may therefore be poly- or monochromatic.

[0019] The photon energy is the characteristic energy of the photon, as defined by its frequency, v, through the relationship E = hv (where h is the Planck constant). As such, the photon energy is not related to the power or flux of the light source being used for treatment.

[0020] The bandgap of the semi-conducting phase is the energy difference between the highest energy state in the (filled) valence band and the lowest energy state in the (un-filled) conduction band.

[0021] The material is treated with light as described above in the presence of a reducing agent. A reducing agent here is any entity capable of reducing the transition metal-based material, such that electrons are donated to the transition metal-based material from the reducing agent.

[0022] As is described in detail below in the detailed description, the treatment with light of photon energy greater than the bandgap dramatically accelerates electron donation by the reducing agent, and as such brings about phase change of the semi-conducting phase to a metallic phase in much reduced time. The method as described above is simple to implement, safe and effective.

[0023] The method allows for spatio-selective phase patterning of layered transition metal-based materials by virtue of the much accelerated phase change. Spatio-selectivity may be created by spatially-confined illumination (for example by treatment with a small cross-section laser, interference lithography, or otherwise), or by masking the material from illumination (for example with a shadow mask).

[0024] Advantageously, the method is safe, and does not require harsh chemical or physical conditions in order to bring about phase change.

[0025] Advantageously, it may be that the method creates a more uniform phase change across the material being treated. It is thought that the light treatment can allow for intercalation through the surface of the layered transition metal-based material, leading to less irregularity in the material and a more uniform distribution of metallic phase.

[0026] It may be that > 75%, > 80%, > 85%, > 90%, > 95%, > 99%, or > 99.9% of the semi-conducting phase of the layered transition metal-based material that is treated with light of photon energy that is greater than the bandgap of the semi-conducting phase in the presence of a reducing agent is converted to the metallic phase. Such conversion efficiency I uniformity may be confirmed by, for example, direct crystallographic imaging using transmission electron microscopy. Such conversion efficiency I uniformity may also be confirmed by, for example, the emergence of Raman J peaks exhibited by the metallic phase (Raman peaks at about 150 cm'1, 225 cm'1, and 315 cm'1). Such a conversion efficiency I uniformity may also be confirmed by, for example, taking a photoluminescence measurement before and after treatment and observing the change in response produced by the material visually or with computer software. In view of the fact that the metallic phase can be characterised by a lack of PL response (i.e. a lack of significant or measurable PL response), the conversion efficiency / uniformity may be characterised by < 10%, < 5%, <1%, < 0.1% of a measured area producing a PL response (i.e. a measurable or significant PL response). It may be that the layered transition metal-based material obtained or obtainable by the method of the invention consists of > 75%, > 80%, > 85%, > 90%, > 95%, > 99% or > 99.9% metallic phase or is substantially pure metallic phase. It may be that an area of the transition metal-based material that has been treated according to the present invention comprises, if present at all, any non- metallic (e.g. semi-conducting) regions in a uniform distribution. It may be that there is substantially no wrinkling observed in a material obtained or obtainable by the methods of the invention. Wrinkling describes the presence of small regions of semi-conducting phase in amongst the metallic phase, creating numerous phase boundaries in the material.

[0027] It may be that the layered transition metal-based material obtained or obtainable by the methods of the present invention is a multi-layer material. It may be that the layered transition metal-based material obtained or obtainable by the methods of the present invention comprises two or more (i.e. at least 2) layers.

[0028] In a second aspect, the present invention provides a method of phase patterning comprising the method of the first aspect, wherein a first region of the semi-conducting phase of the layered transition metalbased material is illuminated, but a second region of the semi-conducting phase of the layered transition metal-based material is not illuminated, such that only the first region is converted into the metallic phase.

[0029] It may be that the second aspect of the present invention is implemented by either of: i) a shadow-mask is used to mask a first region of the semi-conducting phase of the layered transition metal-based material and to allow illumination of a second region of the semi-conducting phase of the layered transition metal-based material, such that only the second region is converted into the metallic phase; or ii) spatially-confined illumination is used to illuminate a first region of the semi-conducting phase of the layered transition metal-based material, but not illuminate a second region of the semi-conducting phase of the layered transition metal-based material, such that only the first region is converted into the metallic phase.

[0030] Phase patterning is achieved by selective phase change of chosen semi-conducting regions of the layered transition metal-based material. Phase patterning is particularly useful for the production of electronic and optoelectronic devices such as transistors, diodes, RF devices, photovoltaics, sensors, detectors and the like. Principally, phase patterning creates any desired pattern of conducting regions on the material, which can benefit many areas of technology. The patterning can also be useful in the production of powdered materials that can be used as lubricants, reinforcements (for plastic or rubber), catalysts, electrochemical (for example, battery) materials, and the like.

[0031] Phase-patterning is achieved by spatio-selective illumination. The greatly accelerated phase change achieved by illumination with light of a photon energy above the band gap of the semi-conducting material allows for, effectively, only the regions of the semi-conducting phase which are treated with the light to be converted to a metallic phase on the timescale of the reaction.

[0032] Spatio-selective illumination can be achieved by either the use of a shadow-mask (i.e. a material that is optically opaque to the light with photon energy greater than the bandgap of the semi-conducting material). Spatio-selective illumination can also be achieved by spatially-confined illumination (i.e. by using a precise laser, for example with a small cross-section, or through interference lithography, or otherwise).

[0033] In a third aspect, the present invention provides a layered transition metal-based material obtained or obtainable by the method of the first aspect, or the second aspect of the present invention.

[0034] The benefits and uses of material produces by the methods of the first and second aspects of the present invention are set out above.

[0035] Further optional features of the invention will now be set out. These can be applied singly or in any combination with any aspect of the invention unless context demands otherwise.

[0036] In some embodiments, the layered transition metal-based material comprises a transition metal within any one of groups 4-11. It may be that the transition metal is in one of periods 4, 5 or 6 of the periodic table. It may be that the transition metal is within group 4. It may be that the transition metal is within group 5. It may be that the transition metal is within group 6. It may be that the transition metal is within group 7. It may be that the transition metal is within group 8. It may be that the transition metal is within group 9. It may be that the transition metal is within group 10. It may be that the transition metal is within group 11. It may be that the transition metal is in period 4. It may be that the transition metal is in period 5. It may be that the transition metal is in period 6.

[0037] In some embodiments, the layered transition metal-based material comprises a transition metal within group 6.

[0038] In some embodiments, the layered transition metal-based material comprises a transition metal that is molybdenum (Mo) or tungsten (W).

[0039] In some embodiments, the layered transition metal-based material is a layered transition metal dichalcogenide material. A chalcogenide is a group 16 element, and as such a transition metal (M) dichalcogenide (X) has the stoichiometry MX2.

[0040] In some embodiments, the layered transition metal dichalcogenide material is a layered transition metal oxide, layered transition metal sulphide, layered transition metal selenide or layered transition metal telluride. It may be that the layered transition metal dichalcogenide material is a layered transition metal oxide. It may be that the layered transition metal dichalcogenide material is a layered transition metal sulphide. It may be that the layered transition metal dichalcogenide material is a layered transition metal selenide. It may be that the layered transition metal dichalcogenide material is a layered transition metal telluride.

[0041] It may be that the layered transition metal-based material is a molybdenum dichalcogenide. It may be that the layered transition metal-based material is a tungsten dichalcogenide. It may be that the layered transition metal-based material is M0S2. It may be that the layered transition metal-based material is MoSe2. It may be that the layered transition metal-based material is MoTe2. It may be that the layered transition metal-based material is WS2. It may be that the layered transition metal-based material is WSe2. It may be that the layered transition metal-based material is WTe2.

[0042] In some embodiments, the semi-conducting phase is a trigonal prismatic phase and the metallic phase is an octahedral phase. The geometry of the semi-conducting and metallic phases refers to the coordination environment of the transition metal. The change in geometry of the layered transition metalbased material is associated with a change in electronic band structure, and thus a change in electronic properties.

[0043] Here, trigonal prismatic phase includes trigonal prismatic phase, distorted trigonal prismatic phase, or a combination thereof. It may be that the trigonal prismatic phase is a 1H phase, a 2H phase, or a combination thereof. Herein, the semi-conducting phase may be referred to as 2H for ease, but this does not strictly limit the teaching of the specification to only the 2H semi-conducting phase.

[0044] Here, octahedral phase includes octahedral phase, distorted octahedral phase, or a combination thereof. It may be that the metallic phase is a 1T phase, a distorted 1T phase, or a combination thereof. That is, it may be that the metallic phase is a 1T phase, 1T’ phase, 1T” phase, dT phase, DT phase, Td phase, or a combination thereof. Herein, the metallic phase may be referred to as a 1T phase for ease, but this does not strictly limit the teaching of the specification to a metallic phase consisting only of 1T. Alternatively, the metallic phase may be referred to as a 1T / 1T’ phase for ease, again, this does not strictly limit the teaching of the specification to a metallic phase consisting only of 1T and 1T’ phases.

[0045] In some embodiments, the layered transition metal-based material is in a physical state selected from one or more of bulk, powder, granules, multi-layer, bilayer or monolayer exfoliated from crystals or grown by chemical vapor deposition methods. It may be that the layered transition metal-based material is in a bulk physical state. It may be that the layered transition metal-based material is in a powdered physical state. It may be that the layered transition metal-based material is in a granular physical state. It may be that the layered transition metal-based material is in a multi-layered physical state. It may be that the layered transition metal-based material is in a bilayer physical state. It may be that the layered transition metalbased material is in a monolayer physical state. The physical state chosen will reflect the use case of the material produced by a method of the invention. For example, mono-, bi- or multi-layer materials may be used in electronic devices, whilst powdered materials may be used as lubricants or in electrochemical (battery) devices. In some embodiments, the reducing agent comprises a metal capable of intercalation of the layered transition metal-based material. As explained in the detailed description, in some embodiments electron donation by the reducing agent is associated with intercalation of a metal between the layers of the material. During intercalation the metal inserts into the inter-layer gaps and associates with adjacent layers of the layered transition metal-based material. It may be that the metal capable of intercalation the layered transition metal-based material intercalates in cationic form. The reducing agent capable of intercalation will form a product with some fractional or integer stoichiometry in the final product. For example, in the case where the layered transition metal-based material is M0S2, and a reducing agent is n-BuLi, the metallic phase will have a stoichiometry represented by LixMoS2. When the layered transition metal-based material is a monolayer, there can be no intercalation. In this case, the metal is best described as associating with the monolayer. As such, herein, “capable of intercalation of the layered transition metal-based material” for the case of a monolayer transition metal-based material should be interpreted as “capable of intercalation of the layered transition metal-based material if it were in multilayer form”.

[0046] The increased spacing between the layers of the material weakens the interlayer attraction, and makes exfoliation of layers of the material easier. As such, it may be that the method of phase conversion according to the present invention can also be used to facilitate fast and simple exfoliation of layered transition metal-based materials.

[0047] In some embodiments, the reducing agent comprises a group 1, group 2 or group 13 metal. It may be that the metal is a group 1 metal. It may be that the metal is a group 2 metal. It may be that the metal is a group 13 metal. It may be that the metal is selected from the group consisting of: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). It may be that the metal is selected from the group consisting of Li, Na, K, Mg and Ca. It may be that the metal is selected from the group consisting of: Li, Na and Ca. It may be that the metal is selected from the group consisting of: Li and Na. It may be that the metal is Li. It may be that the metal is Na. It may be that the metal is Ca. It may be that the reducing agent comprises a group 2 metal. In some embodiments, the reducing agent comprises calcium. It may be that the reducing agent comprises Mg. It may be that the reducing agent comprises Sr. It may be that the reducing agent comprises Ba.

[0048] In some embodiments, the reducing agent is an organometallic reducing agent. Here, organometallic reducing agent refers to an entity comprising an organic component bonded to a metallic component. The organic component may be described as anionic, and the metallic component may be described as cationic. It may be that the organometallic reducing agent is an organolithium reducing agent. It may be that the organometallic reducing agent is an organosodium reducing agent. It may be that the organometallic reducing agent is an organocalcium reducing agent. It may be that the reducing agent is an alkyllithium reagent. It may be that the reducing agent is an alkylsodium reagent. It may be that the reducing agent is an alkylcalcium reagent. It may be that the reducing agent is a butyllithium reducing agent (for example, n-BuLi or t-BuLi). When the reducing agent comprises calcium, the calcium may act as an intercalant. This may be the case when the transition metal-based material comprises two or more layers. In such a scenario, the layered transition metal-based material obtained or obtainable by the method of the present invention may be intercalated with a calcium ions, and have a formula of 1T-CaxTM, where TM is the layered transition material-based material formula without intercalant (e.g. M0S2), and x > 0 and may be fractional or an integer.

[0049] In some embodiments, the reducing agent is an aromatic organometallic reducing agent. Here, the organic component of the reagent comprises an aromatic ring. It may be that the organic component of the reagent comprises a monocyclic aromatic system. It may be that the organic component of the reagent comprises a bicyclic aromatic system. It may be that the organic component of the reagent comprises a tri- or multi-cyclic aromatic system. It may be that the organic component of the reagent is substituted. It may be that the aromatic organometallic reducing agent comprises an organic component selected from the group consisting of: phenanthrene, Anthracene, Pyrene, Triphenylene, Tetracene, Biphenyl, Napthalene and Perylene.

[0050] A particular benefit of the use of aromatic organometallic reducing agents is the ability to tune the redox potential of the reagent by altering the organic component, and the metallic component. As explained in the detailed description, it can be preferable to ensure that the redox potential of the reducing agent is between the potential for the phase change and the potential for a decomposition reaction. Specifically, it may be that the redox potential of the reducing agent is near that for phase change, but more positive than that for decomposition.

[0051] As such, it may be that the redox potential of the reducing agent is close to the redox potential for the phase change from a semi-conducting phase to a metallic phase, for example it may be that the redox potential of the reducing agent is within 0.2 V of the redox potential of the phase change, or within 0.1 V, or within 0.05 V. It may be that the redox potential of the reducing agent is not lower than the redox potential for the decomposition of the layered transition metal-based material.

[0052] The redox potentials of the phase conversion, and the decomposition, of the layered transition metalbased material can be varied by changing the metal of the reducing agent, and the constituents of the layered transition metal-based material itself.

[0053] It may be that the metallic phase of the layered transition metal-based material is thermodynamically stable, or meta-stable, with respect to a decomposition product.

[0054] In some embodiments, the light has a minimum power density of 20mW / cm2. It may be that the light has a maximum power density of 1x106mW / cm2.

[0055] If the layered transition metal-based material is M0S2 the minimum photon energy of the light under is approximately 2 eV. If the layered transition metal-based material is MoSe2, the minimum photon energy of the light is approximately 1.7 eV. If the layered transition metal-based material is WS2, the minimum photon energy of the light is approximately 2 eV. If the layered transition metal-based material is WSe2, the minimum photon energy is approximately 1 .5 eV. It may be that the light used to treat the semi-conducting phase has a photon energy at least 0.1 eV greater than the band gap, preferably at least 1 eV greater than the band gap, more preferably at least 1.5 eV greater than the band gap. It may be that light of a higher photon energy accelerates phase change relative to light of a lower photon energy, where both photon energies are greater than the band gap of the semi-conducting phase.

[0056] In some embodiments, the light has a minimum wavelength of about 220 nm. It may be light with a wavelength below 220 nm is absorbed by a solvent. This is undesirable. It may be that the light has a minimum wavelength of about 230 nm, or about 240 nm, or about 250 nm, or about 300 nm.

[0057] In some embodiments, the method of phase patterning is carried out using light of power 1 W / cm2or less. It is a notable advantage that the claimed method of phase patterning requires significantly less energy and power than known prior art methods, and powers as low as 1 W / cm2or less can be used. It may be that the method of phase patterning is carried out using light of power 0.75 W / cm2or less. It may be that the method of phase patterning is carried out using light of power 0.5 W / cm2or less. It may be that the method of phase patterning is carried out using light of power 2 W / cm2or less. It may be that the method of phase patterning is carried out using light of power 5 W / cm2or less. It may be that the method of phase patterning is carried out using light of power 10 W / cm2or less.

[0058] It may be that the method is carried out in an organic solvent. It may be that the organic solvent is selected from the group consisting of hexane, THF and 1 ,2-Dimethoxyethane.

[0059] It may be that the temperature and pressure used in the methods of the present invention are ambient. It may be that the temperature is above ambient in order to aid the kinetics of the phase change. It may be that the reducing agent has a concentration of from 0.1 to 2.5 M. It may be that a saturated concentration of aromatic reducing agent is between 0.01 and 0.5 M.

[0060] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0061] Summary of the Figures

[0062] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0063] Fig. 1 illustrates light driven phase change of M0S2 during chemical lithiation. Optical setup for the studies described herein. Illumination with 455 nm accelerates the phase transition by up to 2 orders of magnitude.

[0064] Fig. 2 illustrates light driven phase change of M0S2 during chemical lithiation. Schematic diagram for light-driven 2H to 1T / 1T’ phase transition of TMD. Illumination with 455 nm accelerates the phase transition by up to 2 orders of magnitude. Fig. 3 illustrates light driven phase change of M0S2 during chemical lithiation. Image of mono- (1 L) and bi-layer (2L) 2H-M0S2 and 1T-LixMoS2 under different illumination wavelengths. The 2H phase (left images) changed to 1T phase (right images) in 698 min under illumination with 730 nm (top), and 21 min under with 455 nm (bottom, scale bar = 5 pm).

[0065] Fig. 4 illustrates light driven phase change of M0S2 during chemical lithiation. PL spectra of 2L 2H-M0S2 (as exfoliated) and IT-M0S2 produced using three different methods: 48 h n-BuLi immersion in the dark, 24 h of 730 nm (below band gap) illumination, and 25 min of 455 nm (above band gap) illumination. It is noted that that the higher optical resolution achieved at 455 nm reveals more fine structure in the flake, see Fig. 28 for comparison of the same flake under 455 nm and 730 nm.

[0066] Fig. 5 illustrates light driven phase change of M0S2 during chemical lithiation. Raman spectra of 2L 2H- M0S2 (as exfoliated) and IT-M0S2 produced using three different methods: 48 h n-BuLi immersion in the dark, 24 h of 730 nm (below band gap) illumination, and 25 min of 455 nm (above band gap) illumination. It is noted that that the higher optical resolution achieved at 455 nm reveals more fine structure in the flake, see Fig. 28 for comparison of the same flake under 455 nm and 730 nm.

[0067] Fig. 6 illustrates a mechanistic explanation of light-driven phase transition of M0S2 during chemical lithiation. Snapshots of the phase transition visualized at 730 nm. Scale bar = 5 pm.

[0068] Fig. 7 illustrates a mechanistic explanation of light-driven phase transition of M0S2 during chemical lithiation. Histogram analysis for 1L and 2L region.

[0069] Fig. 8 illustrates a mechanistic explanation of light-driven phase transition of M0S2 during chemical lithiation. Selected reaction dynamics from two different points of the 2L indicated in Fig. 6 Both spots are located a similar distance (1 pm) from the edge.

[0070] Fig. 9 illustrates a mechanistic explanation of light-driven phase transition of M0S2 during chemical lithiation. Snapshots of the phase transition visualized at 455 nm. Scale bars = 5 pm.

[0071] Fig. 10 illustrates a mechanistic explanation of light-driven phase transition of M0S2 during chemical lithiation. Histogram analysis for 1L and 2L region.

[0072] Fig. 11 illustrates a mechanistic explanation of light-driven phase transition of M0S2 during chemical lithiation. Power dependence of the reaction dynamics under 455 nm laser illumination for three different flakes. Each curve was obtained 1 urn away from the 2L / glass edge of the studied sample.

[0073] Fig. 12 illustrates the correlation of phase transition by chemical and electrochemical lithiation. Chemically-induced phase transition of M0S2 and WS2 observed in RICM (Scale bar = 5 pm).

[0074] Fig. 13 illustrates the correlation of phase transition by chemical and electrochemical lithiation. Reaction dynamics at selected points indicated in Fig. 12 for M0S2 and WS2 during chemical lithiation. No data was recorded for WS2 between -220-250 min.

[0075] Fig. 14 illustrates the correlation of phase transition by chemical and electrochemical lithiation. Energy landscape proposed for the phase transition and conversion reaction of M0S2 and WS2. The glowing line shows the light-activated pathways. Arrows at the conversion barriers indicate the thermodynamic preference of the system.

[0076] Fig. 15 illustrates photo-redox phase patterning on M0S2. Experimental scheme for photo-redox phase patterning of a 1T phase pattern (left), (right) Image of patterned flake and average across the red rectangle to show the profile of the patterned 1T phase.

[0077] Fig. 16 illustrates photo-redox phase patterning on M0S2 Response comparison of non-patterned (2H), and phase patterned (1T / 2H / 1T) photodetector.

[0078] Fig. 17 illustrates a schematic of the optical cell used herein.

[0079] Fig. 18 illustrates a schematic of the experimental procedure used herein.

[0080] Fig. 19 illustrates contact fabrication for photodetector (scale bar = 20 pm)

[0081] Fig. 20 illustrates PL (top) and Raman (bottom) for M0S2 (left) and WS2 (right) with different numbers of layers.

[0082] Fig. 21 illustrates PL (top) and Raman (bottom) for MoSe2 (left) and WSe2 (right) with different numbers of layers.

[0083] Fig. 22 illustrates layer dependent Raman signature of IT-M0S2.

[0084] Fig. 23 illustrates Raman graph of bilayer IT-M0S2 with different washing step (Hexane washed sample refers to 1T / 1T’-LixMoS2 and DI / Acetone / IPA washed sample refers to IT-M0S2).

[0085] Fig. 24 illustrates an optical image of 2H-M0S2 (scale bar = 10 pm).

[0086] Fig. 25 illustrates an optical image of IT-M0S2 (scale bar = 10 pm).

[0087] Fig. 26 illustrates reflectance spectra of the 1T and 2H phase of M0S2.

[0088] Fig. 27 illustrates optical images of M0S2 with different illumination energy. A: 455 nm (above the bandgap). B: 730 nm (below the bandgap) illumination (scale bar = 5 pm).

[0089] Fig. 28 illustrates the phase transition mechanism of bi-layer M0S2 by steady-state PL analysis. Specifically, PL spectra of 2H, 1T and sample treated with n-BuLi for 1 h.

[0090] Fig. 29 illustrates the phase transition mechanism of bi-layer M0S2 by steady Raman analysis. Raman spectra of 2H, 1T and 1 h n-BuLi treated sample.

[0091] Fig. 30 illustrates the phase transition mechanism of bi-layer M0S2. Schematics of initial dynamics for phase transition.

[0092] Fig. 31 illustrates the wavefront speed for different illumination conditions.

[0093] Fig. 32 illustrates Raman spectra of 1T phase of M0S2 and WS2.

[0094] Fig. 33 illustrates optical images of 1T phase of M0S2 and WS2 (scale bar = 20 pm). Fig. 34 illustrates a snapshot of phase transition dynamics measured in RICN at different time and steady-state PL and Raman spectra of as-exfoliated, illuminated, and ex-situ (48h n-BuLi treatment in dark) bilayer MoSe2 (scale bar = 5 pm).

[0095] Fig. 35 illustrates a snapshot of phase transition dynamics measured in RICN at different time and steady-state PL and Raman spectra of as-exfoliated, illuminated, and ex-situ (48h n-BuLi treatment in dark) bilayer WS2 (scale bar = 5 pm).

[0096] Fig. 36 illustrates a snapshot of phase transition dynamics measured in RICN at different time and steady-state PL and Raman spectra of as-exfoliated, illuminated, and ex-situ (48h n-BuLi treatment in dark) bilayer WSe2 (scale bar = 5 pm).

[0097] Fig. 37 illustrates ex-situ PL from the non-illuminated and illumination edge for bilayer WX2 (S= S, Se). Top = Schematics of initial dynamics for phase transition. Bottom left = PL of Nonilluminated bilayer (exposed to n-BuLi in dark), Illuminated edge, and exfoliated monolayer of WS2 Bottom right, PL of Nonilluminated bilayer, Illuminated edge, and exfoliated monolayer of WSe2.

[0098] Fig. 38 illustrates Raman analysis of phase transition of monolayer M0S2 with photo-redox phase- patterned flake.

[0099] Fig. 39 illustrates Raman analysis of phase transition of bilayer M0S2 with photo-redox phase-patterned flake.

[0100] Fig. 40 illustrates PL imaging of phase patterned flake (scale bar = 5 pm).

[0101] Fig. 41 illustrates time-dependent photoresponse of pristine 2H-M0S2 with different incident light power. Photoresponse lines from top to bottom: 52.1 uW; 23.1 uW; 9.4 uW; 4.5 uW; 1.5 uW; 0.5 uW.

[0102] Fig. 42 illustrates time-dependent photoresponse of phase-engineering M0S2 with different incident wavelength and light power. Repeatability at 488 nm, 405 nm excitation.

[0103] Fig. 43 illustrates time-dependent photoresponse of phase-engineering M0S2 with different incident wavelength and light power. Photoresponse with different incident light wavelength.

[0104] Fig. 44 illustrates time-dependent photoresponse of phase-engineering M0S2 with different incident wavelength and light power. Photoresponse with different incident light power. Photoresponse lines from top to bottom: 50 uW / cm2; 28.2 uW / cm2; 10.03 uW / cm2; 4.94 uW / cm2; 1.28 uW / cm2.

[0105] Fig. 45 illustrates responsivity comparison of phase patterned (1T / 2H / 1T, square) and non-patterned (2H, triangle) M0S2. The patterned photodetector always shows a higher responsivity. The y axis is response in log scale.

[0106] Fig. 46 illustrates a Raman spectrum of 2H M0S2 before treatment and after treatment with various polycyclic aromatic hydrocarbon (PAH) - cation complexes. The four treatments using anthracene lithium (Anth- Li), anthracene sodium (Anth-Na), pyrene lithium (Pyr-Li), and pyrene sodium (Pyr-Na) shows 1T signature. This is because they have good redox matching. The perylene sodium (perly-Na) which has higher redox potential does not work and Naphthalene sodium (Naph-Na) which has lower redox potential decomposed the M0S2.

[0107] Fig. 47 shows the image of expanded M0S2 crystal in anthracene-sodium treatment (left) and an image of exfoliated thin flakes, which has 1T Raman signature as in Fig. 52 (right).

[0108] Fig. 48 illustrates (left) and optical image of 2H M0S2 before treatment, and the material after treatment with anthracene sodium for 10 minutes with no illumination.

[0109] Fig. 49 illustrates (left) and optical image of 2H M0S2 before treatment, and the material after treatment with anthracene sodium for 10 seconds with illumination.

[0110] Fig. 50 illustrates a Raman spectrum of 2H MoSe2 before treatment with anthracene sodium, and 1T M0S2 obtained after treatment. No illumination.

[0111] Fig. 51 illustrates a Raman spectrum of 2H WS2 before treatment and after treatment with various polycyclic aromatic hydrocarbon (PAH) - cation complexes.

[0112] Fig. 52 illustrates a picture for 2H, and 1T (starting from M0S2 powder with 2, and 6 pm size powder) M0S2 colloidal solution in deionized water (DI) in left. The right graph shows the Raman spectra of two 1T samples.

[0113] Detailed Description of the Invention

[0114] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. The teaching of the specific examples below can be generally applied across the whole scope claimed.

[0115] Photo-Redox Phase Engineering of Transition Metal Dichalcogenides

[0116] While the overall chemical lithiation mechanism has been established, it remains unclear how this phase transition from a semi-conducting phase to a metallic phase occurs on the relevant nano- to micrometer length scale [2]. Recent in-situ electron microscopy studies using either direct lithium contact or electrochemical lithiation revealed the rearrangements occurring during this phase transition on the atomic length scale [27-30]. Similarly, electrochemically-controlled optical differential interference microscopy identified the dynamic propagation of the new metallic phase (1T / 1T’) from the edge of the flake to the center on the micrometer scale

[0031] . Despite this progress, a fundamental real-time nanoscale picture of the semi-conducting to metallic (for example, 2H to 1T / 1T’) phase transition under the action of chemical lithiation agents, such as n-BuLi, is still missing. This hampers rational attempts to accelerate the reaction and improve yields for applications.

[0117] Here, the inventors uncover in real time how the chemically-induced 2H to 1T / 1T’ phase transition from mechanically exfoliated mono- and bi-layer TMD flakes proceeds in n-BuLi by using in-situ optical reflectance interferometric contrast microscopy (RICM). The inventors exploit the different dielectric properties of the 2H and 1T / 1T’ phases to directly quantify the phase transition dynamics on the nanoscale with diffraction limited resolution (-200 nm). Critically, the inventors discover that the phase transition in M0S2 and MoSe2 can completed in just a few minutes if illuminated with above bandgap photons at 455 nm, accelerating the reaction by over two orders of magnitude.

[0118] In contrast, for WS2 and WSe2 continuous illumination triggers an irreversible conversion (decomposition). By combining our optical results with ex-situ Raman and photoluminescence (PL) microscopy measurements, as well as electrochemical studies, the inventors demonstrate a new photo-redox activated reaction pathway which overcomes the otherwise slow charge-transfer limited reaction occurring under ‘dark’ conditions. The inventors validate the newly discovered photo-redox reaction by rapidly phase patterning a 1T / 2H / 1T photodetector based on monolayer M0S2.

[0119] Fig. 1 summarizes the experimental setup used to monitor in-situ the chemically-induced phase transition from the semiconducting 2H to the metallic (quasi-metallic) 1T / 1T’ phase (Fig. 2). The inventors note that it has been reported that both distorted 1T’ phase and 1T phase coexist [2, 32-37]. Mechanically- exfoliated semi-conducting 2H-TMD flakes were placed in an air-tight, optically-accessible cell which could be filled with n-BuLi using a syringe pump. The reaction was spatio-temporally resolved with an RICM using LED illumination at 730 and 455 nm (Fig. 17).

[0120] Typical RICM images for mono- and bi-layer M0S2 before and after the phase transition under both illumination conditions are shown in Fig. 3. Independent of the illumination, 1 min after the reaction was initiated, the bi-layer (2L) appeared brightly reflective, while the mono-layer (1 L) was significantly darker with intensity values similar to the glass substrate. After 698 min under 730 nm illumination, the intensity across the 2L was drastically reduced, leaving behind a smooth surface at the edges of the flake, and a more rugged central region characterized by irregular brighter and darker spots. The same flake darkening was observed under 455 nm, but it only took 21 min, corresponding to a 33-fold acceleration of the reaction speed.

[0121] To confirm that the darkened 2L regions correspond to the metallic IT-M0S2 phase, the inventors carried out steady-state PL, Raman and reflectance microscopy measurements. Prior to chemical lithiation, semiconducting 2H-M0S2 exhibited PL at -660 nm (direct bandgap) and -800 nm (indirect bandgap) alongside pronounced Raman peaks at -385 and -407 cm1(E12g and Aigmodes respectively, Fig. 4 and Fig. 5). After the 2L fully darkened, a loss of all PL emission was observed, as well as shifts of the E2g and AigRaman peaks and the emergence of J-peaks in both illumination conditions, indicating successful 1T formation (Fig. 4 and Fig. 5, 730 nm, 455 nm)

[0038] . The inventors verified that the same PL and Raman signatures are observed when exposing semi-conducting 2H-M0S2 to n-BuLi for 48 hours under dark conditions (Fig. 4 and Fig. 5, dark), and that the monolayer region is also successfully converted to 1T (Fig. 18, 20-23) [1 , 38, 39]. In addition, reflectance measurements show the loss of the A- and B-exciton absorption peak in line with the formation of a metallic 1T phase (Fig. 24-26) [35, 36].

[0122] The results confirm that the darkening of the 2L observed in RICM corresponds to the phase transition from semi-conducting 2H-MoS2 to its metallic 1T phase, and can be sensitively monitored in-situ. Furthermore, using below-bandgap illumination at 730 nm allows one to capture the phase transition in M0S2 as it would occur under nominally dark conditions (Fig. 31 ). In contrast, above bandgap excitation at 455 nm coincides with the C-exciton absorption band of 2H-M0S2, suggesting a photo-activated mechanism which drastically accelerates the phase transition

[0040] .

[0123] To reveal the mechanistic origin of the phase transition in M0S2 under different illumination conditions, the inventors analyzed the RICM images as a function of time after n-BuLi addition. Under 730 nm illumination, Fig. 6 shows that the 2L edges become visibly darker after 74 min and form a pronounced and sharp phase boundary between the brighter semi-conducting 2H and darker metallic 1T phase. The build-up of the 1T phase proceeded into the flake at the 2L / 1 L terrace after 147 min, while a similar phase front progression at the 2L / glass edge was reached after 220 min. Once the 1T phase nucleated, the 1T / 2H phase boundary moved into the flake with a constant velocity of -0.1 nm s-1, remaining sharp throughout. Interestingly, after 366 min, brighter and darker regions formed whenever two phase boundaries of different orientation converged. The inventors attribute these regions to microscopic wrinkles arising from a lattice mismatch [27, 37]. Similar ‘phase frustration’ effects were previously observed for phase transitions occurring in lithium cobalt oxide

[0041] . After the wrinkles formed for the first time, the surface morphology of the metallic 1T phase was no longer smooth but instead stayed rugged with brighter and darker features emerging throughout the remaining phase transition.

[0124] To quantify the reaction dynamics at 730 nm, the inventors performed a histogram analysis of the 1 L and 2L regions (see Materials and Methods, below). As shown in Fig. 7 (left), the 1 L exhibited a single-peaked distribution in the histogram map. This feature quickly reduced in intensity to 0.9 over 50 min with no changes to its width, indicating a homogenous surface change during the initial reaction with n-BuLi. Since 1 L cannot intercalate Li-ions, this initial decay suggests a facile electron donation to the surface. After 50 min, the intensity of the feature slowly increased over the rest of the measurement, which the inventors attribute to additional reflection signals generated by side-products formed during the long course of the reaction diffusing through the solution (see Materials and Methods, below).

[0125] Similarly, at the beginning of the reaction the 2L (Fig. 7, right) exhibited a single peaked feature in the histogram map which rapidly decayed to a value of 0.9 over 50 min - the same timescale observed for the 1 L. To confirm that this reduction corresponds to a surface change of the 2L, the inventors carried out PL measurements (Fig. 28), which revealed that the vdW interaction between each layer is weakened within around 50 min. Since the width of the initial feature does not change during this time, the inventors attribute this process to a homogenous surface electron donation, as observed for the 1 L.

[0126] After 50 min, the intense histogram peak at 0.9 reduces in occurrences and is converted into a lower intensity feature at an intensity of 0.7, which becomes prominent after -300 min. This feature signifies the transition of the semi-conducting 2H phase (bright) to the metallic 1T phase (dark) at the edges of the 2L (compare to Fig. 6). After -400 min, a second peak emerges at a lower intensity (-0.6), accompanied by a broadening of both lower intensity features, coinciding with the formation of a wrinkled 1T region in the center of the 2L. To further illustrate the intrinsic variability of the phase transition as observed at 730 nm, Fig. 8 shows intensity profile over time at the 2L / 1 L terrace and 2L / glass edge (2l_Terrace and 2l_Edge, respectively). 2l_Terrace completed the phase transition in just 140 min, while the equidistant 2l_Edge, required 340 min. This discrepancy likely originates from different activation barriers along these two interfaces, highlighting how the inventors’ approach can be utilized to map out intercalation energy barriers in these materials.

[0127] Taken together the inventors identified that the phase transition observed at 730 nm in 2L M0S2 proceeds initially via a rapid surface electron donation from n-BuLi to the whole flake. In case of I L-M0S2, this reaction is sufficient to form the metallic 1T phase. Subsequently, Li-ion intercalation into the 2L causes the 1T phase to form at the edges of the flake with a sharp phase boundary that moves with a constant velocity into the center of the flake. This indicates a charge-transfer limited reaction mechanism. Lastly, once two phase boundaries of different orientation converge, a wrinkled 1T phase is formed. This wrinkling most likely leads to internal strain build-up, resulting in the continued formation of wrinkled 1T phases further into the flake. Generally speaking, metals from other groups can also achieve intercalation

[0086] ,

[0128] The inventors repeated the same analysis for 455 nm illumination. As shown in Fig. 9, after 5 min, the 2L edges darkening and forming a sharp phase boundary between the bright semi-conducting 2H and dark metallic 1T phase can already be observed. After 13 min, however, this phase boundary was significantly more diffuse and the center of the flake appears noticeably darker. After 17 min, no clear phase boundaries could be identified and the flake appeared to darken homogeneously with no signs of wrinkle formation.

[0129] Analysis of the histogram maps in Fig. 10 for the 1 L (left) revealed an identical, albeit accelerated surface reaction compared to 730 nm illumination. Again, the inventors observed an initial homogeneous darkening of the whole 1 L to an intensity of 0.9 in just 5 min, followed by no further changes. The 2L (right) mimicked this intensity reduction in the first 5 min after addition of n-BuLi, confirming that 455 nm illumination accelerates the surface electron donation step of the reaction. Interestingly, after -10 min, the initial distribution splits into two new distributions: The first distribution follows on from the initial one, reducing continuously in intensity to a value of 0.4. The second distribution grew in at an intensity of 0.5 and decayed over the next 10 min to 0.4. After 15 min, both distributions merged and subsequently remained constant. These observations highlight that the metallic 1T phase under 455 nm illumination can be formed in two ways: Pathway one follows the same mechanism observed under 730 nm illumination (albeit faster), whereby Li-ion intercalation from the edges of the flake (lower-intensity distribution) drive the phase transition. The second pathway proceeds via surface penetration of Li-ions in the center of the flake (higher intensity distribution), without the appearance of a phase boundary.

[0130] Mechanistically, these observations suggest a photo-redox activated mechanism. Initially, photoexcitation into the C-exciton of M0S2 at 455 nm must lower the activation barrier for electron donation, to explain the accelerated surface darkening

[0042] . The inventors hypothesize that C-exciton excitation causes efficient surface-hole generation, thereby reducing the electron transfer rate significantly (as seen by the change time of the initial darkening -100 min at 730 nm vs. -5 min at 455 nm) in line with previous transient absorption reports

[0043] . The associated opening of the vdW gap then enables rapid Li-ion intercalation from the edges of the flake. Due to the continued photo-induced hole formation, this process is no longer charge-transfer limited, but becomes more diffusive, causing the phase boundary to broaden. At the same time, regions far away from the edges continue to darken even before the phase boundary can reach them, suggesting that Li-ion intercalation through the surface layer of the 2L becomes an alternative route that is not dependent on lateral Li-ion diffusion through vdW gap [44, 45]. The interplay of these two rapid pathways avoids wrinkle formation, showcasing a secondary positive effect of the illumination.

[0131] To further support this mechanistic picture, the inventors carried out power-dependent measurements on 2L M0S2, varying the illumination fluence at 455 nm between 20, 110 and 510 mW / cm2. Fig. 11 shows the intensity curves for regions located 1 pm away from the 2L / glass edge (also in Fig. 31 ). The inventors find that the photo-redox activated phase transition is indeed power dependent. Lower fluences (20 mW / cm2) approach the shape of the intensity curves observed under 730 nm illumination, indicative of a chargetransfer limited reaction composed of an initial surface change followed by a period of constant intensity, before the phase boundary traverses over region. In contrast, higher fluences (510 mW / cm2), lack clear features of a phase boundary and simply reduce in intensity to a constant value in as little as 8 min (-90- fold acceleration), demonstrating that the process is governed primarily by a diffusion limited reaction profile.

[0132] The results demonstrate the viability of rapidly producing clean, macroscopic flakes of metallic IT-M0S2 by exploiting a photo-redox pathway, which cannot be achieved under dark conditions. Furthermore, by accelerating the reaction chemical safety risks associated with the pyrophoric nature of n-BuLi can be mitigated and side-reactivity could be significantly reduced leading to improved yields.

[0133] Next, the inventors explored if this photo-redox reaction can be used across different TMDs, such as WS2, MoSe2 and WSe2. Here, the discussion will focus on the comparison of M0S2 and WS2, but a full discussion on all samples is provided in the Supplementary Information, below (Fig. 20, 21 , 32-42). Fig. 12 compares the spatio-temporal changes of M0S2 (top) and WS2 (bottom) in the ‘low power’ regime upon addition of n-BuLi at 455 nm illumination. As described previously, 2L M0S2 shows surface darkening together with phase boundary propagation, while I L-M0S2 reduces only reduces in intensity. Conversely, 1 L and 2L WS2 proceed via a homogenous surface darkening mechanism with no visible phase boundaries at any stage during the reaction.

[0134] To further analyze this difference, Fig. 13 shows the intensity curves for comparable 1 L and 2L regions of the respective TMD. While 1 L and 2L M0S2 exhibit a simultaneous, rapid surface darkening followed by a decrease in intensity in 2L M0S2 due to the phase boundary motion, such a behaviour is absent in WS2. 1L WS2 shows a rapid intensity decay within -100 min, which is not matched by any dynamic feature observed in 2L WS2. Instead, 2L WS2 decays initially to an intermediate intensity not observed in 1 L WS2, and then continues to decay slowly over the next 300 min.

[0135] These results highlight that WS2, in contrast to M0S2, proceeds exclusively via a surface-driven reaction mechanism which appears different between 1 L and 2L WS2. The inventors carried our Raman and PL measurements before and after the reaction of WS2 with n-BuLi (Fig. 20, 21 , 34-36). 2H-WS2 displayed the expected PL and Raman peaks. However, after exposure to 455 nm light in the presence of n-BuLi, the Raman spectrum clearly indicated a 2H phase with high signal to noise, but no PL could be observed. Based on these results, the inventors conclude that the chemical lithiation of WS2 under 455 nm photoexcitation does not produce the desired 1T phase, but instead leads to phase conversion to tungsten and lithium sulfides

[0046] . The inventors further attempted to resolve the phase transition in WS2 under below band gap illumination at 730 nm, however, over the time frame of 48 h, no changes to the flakes could be observed, suggesting less favorable energetics for the chemical lithiation via n-BuLi, as compared to M0S2.

[0136] To rationalize these findings, the inventors compared the data with galvanostatic discharge / charge response of M0S2 and WS2 as in [47 - 48]. M0S2 exhibited two plateaus during the galvanostatic discharge process at 1 .1 V, corresponding to the reversible phase transition from 2H to 1T (semiconducting to metallic), and another at 0.57 V, assigned to conversion via irreversible decomposition [17, 18, 47]. Since the redox potential of n-BuLi is 1 V [1 , 55], the charge-transfer limited phase transition in M0S2 proceeds via a small driving force of ~0.1 V. The slow reaction speed is therefore due to a reasonably large activation energy for electron donation. Illumination with 455 nm light increases the (surface) hole concentration and thereby reduces the activation barrier for electron donation, resulting in an accelerated reaction rate. Critically, photo-excitation of M0S2 does not provide enough driving force to activate the conversion reaction at 0.57 V.

[0137] This picture changes for WS2, where the phase transition and conversion potential are close together and located at between 0.75-0.8 V

[0048] . Chemical lithiation is therefore thermodynamically unfavorable, resulting in an extremely slow reaction with low yield after more than a week of n-BuLi soaking at room temperature (Fig. 32, 33). As for M0S2, illumination at 455 nm reduces the activation barrier for electron transfer. However, due to the close proximity of the phase transition and conversion, continuous photoexcitation cannot selectively stop at the 1T phase for WS2. Instead, the conversion reaction is also photoactivated and occurs shortly after the 1T phase has formed. Examination of the MoSe2 and WSe2 corroborates this picture, whereby MoSe2 could be successfully transitioned to the 1T phase, and WSe2 showed conversion behaviour (Fig. 34 - 37) [49,50].

[0138] Taken together, the inventors propose the energy level scheme presented in Fig. 14 to explain the findings. For M0X2 (X = S, Se), the thermodynamic landscape puts the lithiated, metallic 1T phase into a local minimum with respect to its semiconducting 2H phase and decomposition products [34, 56-58]. While the activation barrier for the 2H-1T phase transition is large, photo-excitation at 455 nm effectively reduces this barrier, leading to fast electron donation and Li-ion intercalation. In WX2, the initial electron donation reaction is thermodynamically unfavorable and can only be initiated under 455 nm illumination. However, since the energy difference between metallic 1T and decomposition products is very small, continuous illumination pushes the system too far, preventing controlled 1T formation, ultimately resulting in conversion. A particularly promising application of the discovered photo-redox driven reaction pathway lies in the possibility to encode arbitrary spatial phase pattern into few-layer TMDs. Currently, phase patterning in TMDs is either achieved by direct laser writing or an electron beam lithography [3, 7, 35, 59]. The former method requires specialized femtosecond lasers or extremely high laser powers, while the latter suffers from the need for a precisely controlled electron beam and stable masking reagents as well as using the n-BuLi for several days [3, 7], Here, the inventors make use of the straightforward photo-redox mechanism in M0S2 to inscribe arbitrary phase patterns with diffraction limited edge-resolution. Fig. 15 shows the experimental setup used to create a well-defined T-shaped pattern of the metallic 1T phase into 1L and 2L semi-conducting 2H-M0S2. Using 455 nm illumination at a power density of 510 mW / cm2, the inventors were able to encode the desired phase pattern into semiconducting 1L-MoS2 in 30 s and into semiconducting 2L MoS2 in 13 min (Fig. 15, top right), and achieved a channel width of 3.3 pm with a deconvolved 1T / 2H edge sharpness of o = 161 nm (Fig. 15, bottom right). PL and Raman imaging confirmed the successful phase transition (Fig. 38-40). It is worth noting that these power densities are ~6 orders of magnitude lower than conventional laser-writing approaches.

[0139] To demonstrate the electronic quality of the photo-redox patterned phase, the inventors used the above approach to inscribe a 1T / 2H / 1T phase junction into 1L semi-conducting 2H-M0S2. Using a power density of 1 W / cm2allowed the manufacture of this junction in 10 s. The inventors then deposited gold electrodes onto the 1T phase area to fabricate a simple phase-engineered photodetector (Fig. 19, see Materials and Methods, below). As shown in Fig. 16, the phase-engineered device exhibited enhanced photocurrent density to devices made with 2H-M0S2, showing responsivity of 85 AW-1and repeatability (Fig. 41-45)

[0060] , confirming that the this phase pattering approach creates high- quality metallic 1T phases on semiconducting 2H nanosheets (Table 3). Furthermore, it demonstrates that the diffraction-limited edge resolution does not significantly impede electron mobility at the phase edges

[0060] .

[0140] In summary, the inventors have demonstrated the real-time visualization of the chemical lithiation reaction causing the 2H to 1T (metallic to semi-conducting) phase transition in few-layer TMDs. Using an in-situ optical reflectance interference contrast microscope to acquire detailed images during the reaction, and correlating the results with steady-state and electrochemical characterizations, the inventors provided a detailed mechanistic picture of the reaction on the nanoscale. Critically, by tuning the illumination conditions, the inventors identified a novel, photo-activated reaction pathway which drastically accelerates the phase transition reaction. This made it possible to rapidly inscribe arbitrary phase patters into M0S2 to generate high-quality photodetector junctions in just a few seconds.

[0141] Going forward the inventors believe that the in-situ characterization approach will enable detailed mechanistic insights into a plethora of other next-generation semi-conductor materials undergoing complex dynamic changes. Furthermore, the opportunity to use low-cost visible light to accelerate, guide and optimize phase-engineering of materials will likely find application in many areas where on-demand arbitrary phase patterning must be achieved rapidly and with high yield. This work also paves the way towards rapid wafer-scale phase patterning with visible light, significantly simplifying the scale up of TMD based devices. Aromatic

[0142] A particularly advantageous implementation of the present invention is one where the reducing agent is an aromatic organometallic reducing agent. The structure of the aromatic anion can be adjusted to tune the redox potential of the reducing agent to be placed to bring about phase change, but not decomposition of the layered transition metal-based material.

[0143] Fig. 46 shows a 2H to 1T phase change in M0S2 with various polycyclic Aromatic Hydrocarbons (PAHs)- cation (Li, Na) solvents as a reducing agent. The reduction potential of each PAHs plays its role here. The anthracene and pyrene, which has the appropriate redox potential for phase transition reaction in M0S2 does change the phase. However, perylene, which has higher redox potential did not change the phase and naphthalene, which has too low redox potential drive the reaction to further decomposition. The shift of the E29and Aigpeaks, and the emergence of Ji , J2 and J3 peaks provides evidence of the phase change. Fig. 47 shows a image of 2H to 1T phase change in M0S2 crystal using anthracene sodium (AnNa) as a reducing agent. The expanded crystal shows successful intercalation and layer expansion. This 1T crystal can be exfoliated to thin layers as in the right image.

[0144] Fig. 48 shows the phase change from 2H to 1T in M0S2 using AnNa without any light treatment. The time elapsed between the left image and the right is 10 minutes and the speed of the phase change front is approximately 5 nm / s. Fig. 49 shows the same phase change with light treatment (LED illumination), the time elapsed between the first and second images is 10 seconds and the speed of the phase change front is approximately 5 pm / s (x1000 compared to no illumination).

[0145] Fig. 50 shows the change in Raman spectrum when MoSe2 is treated with AnNa according to the present invention. The shift in E2g and Aigpeaks and the emerging J peaks is evidence of the desired phase change.

[0146] Fig. 51 illustrates a Raman spectrum of 2H WS2 before treatment and after treatment with various polycyclic aromatic hydrocarbon (PAH) - cation complexes. The treatments using anthracene sodium (Anth-Na) and pyrene sodium (Pyr-Na) shows 1T signature. The perylene sodium (perly-Na) which has higher redox potential does net work and Naphthalene sodium (Naph-Na) which has lower redox potential decomposed the M0S2. The shift in E2gand Aigpeaks and the emerging J peaks is evidence of the change.

[0147] Fig. 52 shows a picture for M0S2 colloidal solution in deionized water (DI). 2H M0S2 is not able to be dispersed in DI water as it is hydrophobic. However, 1T M0S2 is hydrophilic. Therefore, dispersion in DI water is one indicator to see if phase has been changed. The left image shows the 2H powder (2 pm size powder) is not dispersed in DI water but both 1T(Anthracene sodium treated, starting from M0S2 powder with 2, and 6 pm size powder) are well dispersed in DI water. This shows this method is working in powder M0S2. The right graph shows the Raman spectra of two 1T samples.

[0148] RECTIFIED SHEET (RULE 91) ISA / EP Information

[0149] 1. Materials and Methods

[0150] 1.1. Cell design, Sample Preparation and Chemical Lithiation Procedure

[0151] The air-tight cell for the experiments discussed herein has been designed in two parts: as chemical reservoir, and base plate with optical window (Fig. 17). The reservoir has volume of ~2 ml, which is enough to monitor slow reaction dynamics for few days avoiding issue from evaporation. The two ports of reservoir part are assembled with tubes, which are connected to syringes. The first syringe is prepared with filling approximately 3 ml of n-butyllithium (n-BuLi, 1.6 M in hexanes, Sigma-Aldrich), and the second empty syringe is prepared as a container for exhaust air (Nitrogen), when injecting chemical inside the cell.

[0152] M0S2, MoSe2, WS2, WSe2 (HQ graphene) had been exfoliated on microscopic cover glass substrate (thickness = 170 ± 5 pm) by mechanical exfoliation using a polydimethylsiloxane (PDMS) tape. The number of layers was confirmed using the steady-state Raman and photoluminescence (PL) as in Fig. 20, 21 and Table 1 and 2. The glass substrate with TMD flake, two parts of the cell with two syringes, and an O-ring (0I .8 mm, EL-CELL) was assembled inside the nitrogen glovebox.

[0153] The cell was placed on a home-build reflection interferometric contrast microscope (RICM). After locating the desired flake, the inventors injected n-BuLi for ~1 min, resulting in partial filling of syringe 2. The n- BuLi level in both syringes was carefully monitored to ensure the flake remained fully immersed during the course of the experiment.

[0154] The polycyclic aromatic hydrocarbon (PAH) - cation complexes has been synthesized by dissolving polycyclic aromatic hydrocarbon (PAH) in organic solvent including THF and further adding metals in solvent.

[0155] 1.2. Experimental Procedure

[0156] Experimental procedure is illustrated in Fig. 18. The inventors compared three experiments under Red, Blue and No (i.e. Dark) illumination. ‘Red’ refers to in-situ monitoring of the reaction using 730 nm light, and the ‘Blue’ to using 455 nm light. ‘Dark’ designates the conventional 48 h treatment of n-BuLi inside vial without any illumination.

[0157] Lithiation of 2H-M0S2 forms 1T-LixMoS2, which is stable in air [61 , 62]. For removing the excess of butyllithium and other organic by-products [1], the inventors first washed the sample using excess amount of Hexane. The washing step was done inside the cell by injecting the Hexane using syringe for avoiding air-exposure and minimize the chance to possible oxidation of surfaces. After steadystate optical characterization (PL and Raman), the inventors rinsed the sample again using excess amount of distilled water (DI), Acetone, and Isopropyl alcohol (IPA) to remove all lithium(1T-MoS2). Fig. 22 shows the layer-dependent the Raman spectra of IT-M0S2. The inventors monitored

[0158] Raman spectra for each washing steps and confirmed that the structural changes for different washing steps are negligible (Fig. 23). For applying polycyclic aromatic hydrocarbon (PAH) - cation solvents, material in flake of powder type immersed in solvent and washed using filtrations.

[0159] The same procedure was applied to all other TMD materials (MoSe2, WSe2, WS2).

[0160] 1.3 Steady-state Optical Characterization

[0161] The microscope steady-state photoluminescence (PL) and Raman measurements were carried out under ambient conditions using a Renishaw inVia Raman microscope using the excitation laser sources of 532 nm. Prior to all measurements, the spectrometer was calibrated using silicon reference sample for correcting the instrument responses. The laser was set to a power of 0.05% (<0.5 pW) focused to the designated point of the flake using a 100* long working distance objective and emission was collected in streamline mode and dispersed by a 1800 l / mm grating.

[0162] Reflection microscopy was performed with a home-built microscope. Briefly, a variable wavelength excitation was provided by a pulsed super continuum white light source (Fianium Whitelaser) coupled to a monochromator (Bentham TMC 300). The sample was imaged, at each wavelength, in reflection geometry onto an EMCCD camera (Photometries QuantEM 512SC) with a 60 x oil immersion objective (UPLFLN60XOI, Olympus). The microscopic PL image was taken on the same setup, using 532 nm excitation and imaging onto the EMCCD camera with a of 660 nm bandpass filter (Thorlabs).

[0163] 1.4. RICM measurement

[0164] 1.4.1 Methods

[0165] RICM measurements were carried out using a home-built microscope setup reported in detail previously [41 , 63]. Additionally, the microscope was equipped with a 455 nm LED (M455L4, Thorlabs) coupled into a single mode fiber. The output of the fiber was collimated with a condenser lens and imaged into the back-focal plane of the objective (UPLXAPO100XO, Olympus) to achieve wide-field illumination. 730 nm illumination was carried out by inserting a 730 nm bandpass filter (FB730- 10, Thorlabs) into the illumination path of a SOLIS-740C (Thorlabs).

[0166] The magnification of the microscope was 167* and confirmed with a resolution target. 2x2 softwarebased pixel-binning, leading to a pixel pitch of ~70 nm / pixel and, depending on the experiment, 10-30 frame-binning was applied to improve the image quality. The effective acquisition frame rates varied between 0.05 Hz (730 nm illumination) to 3 Hz (455 nm illumination, 510 mW / cm2) ensuring sufficient time-resolution to monitor the phase transformation dynamics.

[0167] Throughout the experiment, the sample was kept in focus using a hardware- based autofocus routine

[0041] . No images were recorded during the injection of n-BuLi due to the large pressure fluctuations and associated focus changed of the sample. For the patterning experiments (Fig. 15), a shadow-mask was placed into the illumination path of the microscope such that it was imaged onto the sample. The flake was carefully positioned with respect to the mask in the absence of n-BuLi. Subsequently, the illumination was turned off and n-BuLi was injected as before. After 1 min, the syringe pump was turned off and the 455 nm illumination was switched on to initiate the photo-driven phase transition. The light source was turned off once the intensity of the flake reduced to the 1T-phase value, and the sample was washed immediately afterwards.

[0168] 1.4.2. Analysis

[0169] Recorded image stacks were corrected for sample drift as previously described

[0041] . No other postprocessing was applied.

[0170] For the histogram analysis presented in Fig. 7 and Fig. 10 the image stack was masked to include only 1L or 2L regions, respectively. The first image in each stack served as a reference frame to compute an average intensity value for each masked region. For each image in the stack, each pixel value in the masked image was subsequently divided by this reference intensity value to generate an intensity distribution centered at 1. For each image in the stack a histogram with 120 equally-spaced bins ranging from intensity values of 0.1 - 1.2 was then computed, yielding the histogram plots shown in Fig. 7 and Fig. 10. No normalisation for the illumination intensity was required, due to the high stability of LED light sources (<5% power drop over 5 days continuous illumination).

[0171] 1.5. The increased intensity for long-term Red illumination

[0172] During long-term experiments (>1 h), necessary to capture the phase transition dynamics under 730 nm illumination, the inventors observed noticeable changes to the intensity at the glass / n- BuLi interface of up to 15% over 12 h. These changes match the observed long-term increase observed in Fig. 7 and Fig. 8 and must therefore be correlated. Given that our light sources are stable, the origin of this behaviour must be related to either a change in the refractive index of the n-BuLi solution, or an increased out-of-focus scattering mechanism.

[0173] In addition to this slow intensity increase, long-term imaging at 730 nm reveals several landing events’ of several cluster-like particles. These particles are likely formed in solution reactions involving the highly reactive n-BuLi. The inventors therefore believe that the long-term increase in reflection intensity is most likely associated with out-of-focus scattering, which our wide-field microscope will be sensitive to.

[0174] 1.6. Photodetector fabrication and measurement

[0175] The photodetector was fabricated by metal deposition using a shadow mask. The inventors aligned the shadow-mask on exfoliated IH / 2H-M0S2, followed by photoredox phase engineering and gold contact deposition selectively on the 1T / 1T’ region. The 50nm of gold pad was deposited using electron-beam evaporation.

[0176] Photodetector measurement was realized by illuminating monochrome laser (MCLS1, Thorlabs) onto devices through a customized lens (Mitutoyo) in order to guarantee the incident angle of light to the device to be 90 degrees. The device was electrically connected to a semiconductor analyser (Keithley 4200) to record the photocurrent.

[0177] 2. Supplementary Data

[0178] 2.1. Layer dependence photoluminescence (PL) and Raman signature of 2H-TMDs Fig. 20 and 21 show the Raman and PL signature measured with different number of layer from each 2H- TMDs, exfoliated on microscopic cover glass for this invention. The inventors confirmed the number of layers by comparing the PL spectra reported in previous studies (Table 1 ). Raman signature from each material with different number of the layers also shows the same trend as reported (Table 2). The position is not exactly the same, but slightly shifted from the different substrates.

[0179] Table 1 . PL signature of TMDs with different number of layer.

[0180] Table 2. Raman signature of TMDs with different number of layer.

[0181] 2.2. Raman spectra of IT-M0S2

[0182] Fig. 22 shows the Raman spectra of 1T / 1T’ M0S2 with different number of layer. The Raman J-peaks

[0183] (J 1 ~157 cm'1, J2=223 cm'1, J3=335 cm'1) are getting stronger with increasing the number of layers, while the E12g peak has red-shifted and Aigpeak has blue-shifted in similar range for all layers (E12g=382 cm'1, Aig=406 cm1) [34,38],

[0184] Fig. 23 shows the Raman signature of 1T / 1T’-MoS2 with different washing steps illustrated in Fig. 18. After phase transition in bi-layer M0S2, the inventors firstly washed the sample using Hexane, which remains lithium cation within the 1T structure (1T / 1T’-LixMoS2) [61 , 62]. Then, second step of washing followed using DI, Acetone and IPA for removing lithium (1T / 1T’-MoS2). The two graphs have very similar spectra having same position of E12g and Aigpeak and J-peaks. Therefore, the inventors concluded removing the lithium has negligible effect on structural change in IT-M0S2.

[0185] 2.3. Optical absorption properties of M0S2

[0186] Fig. 24 and 25 show the steady-state optical image of 2H and IT-M0S2 taken with optical microscope using a 100* air-objective. It shows the optical contrast has decreased as it changed to metallic phase. Fig. 26 is the reflectance spectra of 2H and 1T / 1T’ phase, the two excitonic peak from semiconducting properties (-602 nm, -660 nm) has disappeared in 1T / 1T’ phase [35, 36].

[0187] Fig. 27 shows the optical image using two different illumination wavelengths of 455 nm and 730 nm. The image taken at 455nm has a higher contrast than the image taken with 730 nm due to an increased optical absorbance at this wavelength. The optical absorbance peak of M0S2 locates at around 605 nm (B-exciton, 2.05 eV), 660 nm (A-exciton 1.88 eV), and 450 nm (C-exciton, 2.75eV)

[0040] . This optical absorption also could induce optically generates electron which induced to the chemical reaction process

[0042] ,

[0188] 2.4. Initial dynamics of phase transition in M0S2 from steady-state PL and Raman spectra

[0189] Fig. 28 and 29 show the steady-state measurement of initial dynamics happening in 1 h without illumination (dark). In Fig. 7 the darkening of the bilayer flake stays until around 400 min and the wavefront starts to show from 300 min. Also, initial rapid darkening happens within 50 min. Therefore, the inventors carefully treat n-BuLi on bilayer M0S2 for 1 h to examine the structure change at initial stage.

[0190] The PL shows monolayer-like single peak having its maximum around at 1.84 eV (Top). This indicated dramatic weakening of van der Waals interactions between the layers and each layer reacts as independent single layer. Previous reports show strain induces direct to indirect bandgap transition to mono-layer

[0070] , or a red-shift of the two PL peak to the bilayer M0S2

[0071] . However, this PL spectra (middle A) shows a single peak position of 1 .84 eV which corresponds to the recombination of the negative trion

[0072] . Although the PL intensity is very low compared to the intrinsic monolayer because the dominant trion recombination pathway is nonradiative

[0073] , this single PL peak suggests the van der Waals interaction has weakened. In Raman spectra, the 1 h sample shows the shift and broadening of E12g peak and Aigpeaks, due to the charging of the structure

[0034] . However, the J-peaks, which is relating to the distortion of the structure by lithium cation is not showing yet

[0038] .

[0191] In electrochemical lithiation, it has been reported the opening of the layer structure for lithium intercalation starts at 1.5 V vs. Li / Li+

[0019] . This also could be matched with the initial darkening and monolayer-like single PL, correlated with initial expansion of interlayer spacing before the lithium cation gliding through the gap. This steady-state optical characterization result of the first darkening of the flake in Fig. 7 is highly corresponding to the weakening of the van der Waals interactions and prior phase transition of top-most layer, which is also reported as thermodynamically favourable from the calculation

[0057] , 2.5. The wave front speed in dark, and at different energy and power of illumination

[0192] To compare the phase transition dynamics for different experimental conditions, the inventors estimated the phase front speed in the measured bi-layer M0S2 (Fig. 31 ). To this end, the inventors extracted a line profile perpendicular to a glass / flake interface in all cases, and identified the position of the 1T / 2H phase boundary through half intensity point between the two phases. The velocity was retrieved using a linear fit which did not include the first 1 pm of the glass / flake interface to avoid edge effects. The speed of ‘dark’ has been measured with from stroboscopic imaging once every hour. The front speed of dark and red-illuminated is very similar (Dark: 0.08 nm / s , Red (510 mW / cm2): 0.1 nm / s) while 455 nm illumination shows speeding up to 110 nm / s at higher power densities. The inventors note that this approach works up to 110 mW / cm2where the mechanism is still approximately charge-transfer mediated, resulting in a clear phase boundary at early times. At higher powers, the change to a diffusionlimited mechanism prevents accurate results with this analysis and diffusion modelling is required.

[0193] 2.6. 1T-Phase MoSz and I / I / S2

[0194] In Fig. 32 and 33, the lithiation of thick-layer M0S2 and WS2 follows shrinking-core dynamics from the edge, as previously reported

[0036] . The outer-ring region for both M0S2 and WS2 showed J-peaks of Raman similar to synthesized IT-WS2, which indicates the phase changed to 1T [74-76].

[0195] However, this spontaneous phase transition for WS2 in room temperature is very slow (150 h) and rarely observed (very low yield). So, spontaneous phase transition reaction with n-BuLi in room temperature is much less favorable to WS2 than M0S2.

[0196] 2.7. Phase transition and conversion by above-gap illumination (455nm) on 2H-TMDs

[0197] In Fig. 34, the RICM image of MoSe2 shows surface darkening followed by wave front similar to the M0S2. The wavefront arise not only from the edges but also from the inside of the flake, which could be due to the internal structural strain or defect. MoSe2 changed the phase to 1T, as showing the Raman peaks at J1 (106.4 cm-1), J2 (150.7 cm'1), J3 (221.4 cm'1), E12g (289.4 cm'1), at both illuminated and nonilluminated (dark, ex-situ) condition [77, 78, 5]. In PL signal, the illuminated sample showed disappearance of PL as M0S2. Therefore, the photo-redox process worked for phase transition from 2H to 1T phase as similar to M0S2.

[0198] For WS2, the overall-darkening on the flake happened (RICM image in Fig. 12), and it did not show a 1T signal from Raman but only 2H signal with extremely high signal-to noise ratio (compare ex-situ and illuminated (ilium., Fig. 35, centre). At the same time, the illuminated sample only showed disappearance of PL while the PL stays similar up to 150 h at dark. It describes the light driven reaction triggers not only phase transition but also conversion reaction. Also, the existence of PL shows a lower possibility of spontaneous phase transition of WS2 at dark than M0S2.

[0199] In Fig. 36, RICM images of WSe2 display the surface darkening followed by two wave front moving at very closely. And after the lithiation or dark experiment, no 1T signature found by Raman, with disappearance of PL

[0079] . The electrochemical potential for WS2 has two plateau at very close voltage range and WSe2 has single plateau

[0052] . The same redox potential can be measured by CV [51-54]. The very close, or overlapped electrochemical potential WX2 (X=S, Se) could be related to the direct conversion reaction (irreversible decomposition to W and LixS or LixSe) of WX2 at both electrochemical, and chemical lithiation with above-gap illumination. Although the inventors get the similar results of conversion from WX2, the inventors could see the two wave front moving at very closely only at WSe2. The inventors hypothesized this is mainly due to the most ionic nature of sulphides in chalcogen atoms (with the sequence being S>Se>Te)

[0080] .

[0200] 2.8. Electrochemical interpretation of phase transition

[0201] 2.9. Steady-state PL spectra from the illumination edge of WX2 (X=S, Se)

[0202] Fig. 37 shows the PL data from the illumination-edge of bilayer WS2, and WSe2. Compare to the M0S2, the 455 nm illuminated samples show a slightly different coIor at the illumination edge. This spot exhibits monolayer-like PL, while the illuminated area has no PL (conversion reaction already happened) and the non-illuminated area still shows the PL signal of the bi-layer (2H phase, see also Fig. 20 and 21).

[0203] The inventors postulate that when light activates both the phase transition and the conversion reaction, the illumination alters only the phase of the first layer and resulting in a strong monolayer-like PL. Compared to M0S2, which shows monolayer-like PL signal for the whole non-illuminated bilayer (Fig. 20 and 21) by the weakening of interlayer interaction, this localized monolayer-PL signal from WX2 (X=S, Se) explains different initial dynamics close the solid-solution-like reaction.

[0204] 2.10. Photo-redox diffraction limited phase patterning of M0S2

[0205] Fig. 38 and 39 shows the statistics of Raman graph. For both monolayer and bilayer, the full width at half maximum (FHWM) of E2g Raman peak increases and peak position difference between E2g and AigRaman peak gets wider as the 2H phase converts to 1T (Fig. 20-23). The Raman analysis using different exfoliated samples shows this phase transition reaction in dark takes more than 24 h for both mono- and bi- layer. Our phase-patterned flakes shows the same Raman changes happens less than 20min, which shows the effective photo-redox phase engineering.

[0206] Fig. 40 shows the microscopic PL image was taken on the same setup, using a 532 nm excitation and PL was imaged on the EMCCD camera with 660±10 nm bandpass filters for measuring A- exciton PL. The strong uniform PL form non-illuminated monolayer shows the semiconducting properties of non-illuminated region.

[0207] 2.11. Photoresponse of M0S2 photodetector

[0208] All the photodetectors are working at photoconductor mode with VDS = 1 V. Fig. 41-44 illustrate the detailed power and wavelength dependence of 2H-M0S2 and phase engineered-MoS2 photodetectors respectively. Both devices shows strong power- and wavelength-dependent photocurrent values which is in line with literature reports [81-84]. With similar scales of power density, the inventors observe that the phase engineered-MoS2 shows higher photocurrent than the non-treated 2H-M0S2. The photocurrent also shows reproducibility with illumination on / off cycles. The inventors calculated the responsivity I using following equation:

[0209] Where Iphis the photocurrent obtained from the difference between light current and dark current, P is the excitation power density, and S is the active illuminated area.

[0210] As is demonstrated in Fig. 45, R in phase-engineered M0S2 shows up to 10 times of enhancement at low power illumination compared to untreated 2H M0S2, reaching 85 A / W at VDS = 1 V of bias.

[0211] Comparing to literature-reported R (Table 3), the results indicate that a competitive R value could be achieved simply by phase engineering at the contact without gate modulation from the dielectric and high bias voltage. Table 3 - Comparison for photodetector responsivity based on monolayer M0S2. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0212] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0213] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0214] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0215] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0216] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0217] References

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Claims

Claims:1 . A method of converting a semi-conducting phase of a layered transition metal-based material to a metallic phase of the layered transition metal-based material, comprising treating the semi-conducting phase with light of photon energy that is greater than a bandgap of the semi-conducting phase in the presence of a reducing agent.

2. The method of claim 1 , wherein the layered transition metal-based material comprises a transition metal within any one of groups 4-11 .

3. The method of claim 1 or claim 2, wherein the layered transition metal-based material comprises a transition metal within group 6.

4. The method of any one of claims 1 to 3, wherein the layered transition metal-based material comprises a transition metal that is molybdenum or tungsten.

5. The method of any one of claims 1 to 4, wherein the layered transition metal-based material is a layered transition metal dichalcogenide material.

6. The method of claim 5, wherein the layered transition metal dichalcogenide material is a layered transition metal oxide, layered transition metal sulphide, layered transition metal selenide or layered transition metal telluride.

7. The method of any one of claims 1 to 6, wherein the semiconducting phase is a trigonal prismatic phase and the metallic phase is an octahedral phase.

8. The method of any one of claims 1 to 7, wherein the layered transition metal-based material is in a physical state selected from one or more of bulk, powder, granules, multi-layer, bilayer and monolayer.

9. The method of any one of claims 1 to 8, wherein the reducing agent comprises a metal capable of intercalation of the layered transition metal-based material.

10. The method of any one of claims 1 to 9, wherein the reducing agent comprises a group 1 , group 2 or group 13 metal.

11. The method of any one of claims 1 to 10, wherein the reducing agent is an organometallic reducing agent.

12. The method of any one of claims 1 to 11 , wherein the reducing agent is an aromatic organometallic reducing agent.

13. The method of any one of claims 1 to 12, wherein the reducing agent comprises calcium.

14. The method of any one of claims 1 to 13, wherein the light has a minimum wavelength of about 220 nm.

15. A method of phase patterning comprising the method of any one of claims 1 to 14, wherein a first region of the semi-conducting phase of the layered transition metal-based material is illuminated, but a second region of the semi-conducting phase of the layered transition metal-based material is not illuminated, such that only the first region is converted into the metallic phase.

16. The method of phase patterning of claim 15, wherein the power density of the light is less than 1 W / cm2.

17. A layered transition metal-based material obtained or obtainable by the method of any one of claims 1 to 16.

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