PHOTOVOLTAIC CELLS

Abstract

This invention relates to cells and devices for harvesting light. Specifically the cell comprises at least one electrode which comprises graphene or modified graphene and layer of a transition metal dichalcogenide in a vertical heterostructure. The cell may be part of a light harvesting device. The invention also relates to materials and methods for making such cells and devices.

Claims

1.-20. (canceled)

21. A method of converting light energy to electrical energy, the method comprising exposing a photovoltaic cell to light energy, wherein the photovoltaic cell has a light harvesting portion which comprises at least the following layers: a first electrode layer which comprises graphene or modified graphene (e.g. doped graphene); one or more layers comprising transition metal dichalcogenide (TMDC); and a second electrode layer; wherein the layers are stacked sequentially to form a laminate structure and the or each layer of transition metal dichalcogenide is situated between the first and the second electrode layer and the or each TMDC layer is in electrical contact with both electrodes.

22. The method of converting light energy to electrical energy of claim 21, wherein the first electrode layer, the TMDC layer and the second electrode layer each comprise one or more two-dimensional crystals.

23. The method of converting light energy to electrical energy of claim 21, wherein the second electrode layer comprises graphene or modified graphene.

24. The method of converting light energy to electrical energy of claim 21 which further comprises a gate electrode.

25. The method of converting light energy to electrical energy of claim 21 in which one of the electrode layers comprises graphene and the other comprises graphene which has been doped with a dopant which changes the work function of graphene.

26. The method of converting light energy to electrical energy of claim 25, wherein the dopant is not chemically bonded to graphene.

27. The method of converting light energy to electrical energy of claim 21, wherein the only active layers the cell contains are the first and second electrode layers and the layer comprising the TMDC.

28. The method of converting light energy to electrical energy of claim 21, wherein the first electrode layer comprises graphene with metal nanostructures on its surface.

29. The method of converting light energy to electrical energy of claim 28, in which the metal nanostructures comprise Au.

30. The method of converting light energy to electrical energy of claim 21, wherein the or each TMDC layer is from about 20 nm to 50 nm thick.

31. The method of converting light energy to electrical energy of claim 21, wherein the TMDC is selected from the group consisting of MoS.sub.2 and WS.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0106] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings.

[0107] FIG. 1 shows the dimensions of an exemplary polymer stack onto which flakes are exfoliated for transfer.

[0108] FIG. 2 shows the dimensions of an exemplary device.

[0109] FIG. 3 shows the dimensions of the plasmonic nanostructures of an exemplary device.

[0110] FIG. 4 shows a representative optical image of one of a device according to the invention (left hand top panel); a schematic diagram of the device (right hand top panel); and photocurrent maps taken before (above) and after (below) the device was stored in a humid atmosphere (bottom panel).

[0111] FIG. 5 shows IV curves taken at gate voltage values from 20 to +20 V in 10 V steps.

[0112] FIG. 6 shows the IV characteristics of the device shown under laser illumination of varying intensity.

[0113] FIG. 7 shows the extrinsic quantum efficiency of the devices. The inset shows the photocurrent for a device on Si/SiO.sub.2 (open squares) and for a flexible device on PET (crossed squares).

[0114] FIG. 8 shows a comparison of photocurrent signal as a function of gate voltage; before and after thermal evaporation of a 1 nm thick gold film. Taken at zero bias.

[0115] FIG. 9 shows a photocurrent map of the nanostructured device B.

DETAILED DESCRIPTION

[0116] The term two-dimensional heterostructure refers to a plurality of two-dimensional crystals arranged in a stack. A heterostructure comprises at least two different materials. The two-dimensional crystals are arranged such that the heterostructures are substantially parallel, being arranged face-to-face, forming a laminate. Such heterostructures may also be called vertical heterostructures. Various structures may be intercalated between the crystals e.g. nanoparticles, nanotubes, quantum dots and wires. It may be, however, that the heterostructure is formed entirely of two-dimensional crystals. This does not preclude the heterostructure from being mounted on a substrate and/or have a protective coating. Nor does it preclude the possibility that nanostructures are present but are not intercalated between the layers. A two-dimensional heterostructure is so-called because it is comprised of two-dimensional crystals. It will itself, of course, be a three dimensional structure.

[0117] Examples of two-dimensional crystals which may be included in the heterostructures of the invention include graphene, modified graphene (e.g. graphane, fluorographene, chlorinated graphene), BN, MoS.sub.2, NbSe.sub.2, Bi.sub.2Te.sub.3, MgB.sub.2, WS.sub.2, MoSe.sub.2, TaSe.sub.2, NiTe.sub.2.

[0118] Heterostructures may be formed by placing two-dimensional crystals upon one another mechanically, epitaxially, from solution and/or using any other means which would be apparent to the person skilled in the art.

[0119] A graphene heterostructure comprises at least one two-dimensional crystal of graphene or modified graphene.

[0120] The term two dimensional crystal means a crystal which is so thin that it exhibits different properties than the same material when in bulk. Not all of the properties of the material will differ between a two-dimensional crystal and a bulk material but one or more properties are likely to be different. A more convenient definition would be that the term two-dimensional crystal refers to a crystal that is 10 or fewer molecular layers thick, e.g. one molecular layer thick, but this depends on the material. Crystals of graphene which have more than 10 molecular layers (i.e. 10 atomic layers: 3.5 nm) generally exhibit properties more similar to graphite than to graphene. A molecular layer is the minimum thickness chemically possible for that material. In the case of graphene one molecular layer is a single atom thick. The same is true of boron-nitride. In the case of the transition metal dichalcogenides (e.g. MoS.sub.2 and WS.sub.2), a molecular layer is three atoms thick (one transition metal atom and two chalcogen atoms). Thus, two-dimensional crystals are generally less than 50 nm thick, depending on the material and are preferably less than 20 nm thick. Graphene two-dimensional crystals are generally less than 3.5 nm thick and may be less than 2 nm thick.

[0121] The term two dimensional crystal includes crystals which are doped, as described below.

[0122] The term modified graphene refers to a graphene-like structure that has been modified in some way. Thus, the modified graphene may be graphene which has been doped. This may have the purpose of modifying the work function of graphene without significantly reducing its conductivity. Examples of compounds which can be used to dope graphene are: NO.sub.2, H.sub.2O and I.sub.2, which act as acceptors to provide a p-doped graphene; or NH.sub.3, CO and C.sub.1-C.sub.3 alcohols (e.g. ethanol), which act as donors to provide an n-doped graphene. Small amounts of doping can increase the transparency of the doped graphene relative to graphene but the dopant itself may absorb or reflect light. Conventional methods of doping the graphene can be used to improve the functionality of the graphene, including its transparency to actinic radiation. These methods of doping are described in the literature and are not therefore reproduced here. An alternative approach to doping is to place metal (e.g. gold) nanostructures on the surface of the graphene. This will both dope the graphene and increase the local electric field. A preferred dopant is one which is not chemically bonded to graphene but which is able to transfer charge to graphene, effectively altering the graphene's work function.

[0123] When graphene is placed in contact with a transition metal dichalcogenide layer, there is a dopant effect, i.e. the graphene's work function is changed. Thus, for the absence of doubt, throughout this specification, whenever graphene is referred to as being doped, this means that it is doped by something in addition to the TMDC.

[0124] An electrode layer is a layer of a material which is an electrical conductor.

[0125] A heterostructure is a structure comprising two or more different materials. The materials may be arranged in any way in relation to each other.

[0126] As use in this specification, a layer of a material refers to a plane of that material.

[0127] Each layer may comprise any number of molecular layers of the same chemical composition. Thus a layer of graphene does not necessarily mean a graphene monolayer, although it might. Likewise, a layer of WS.sub.2 does not necessarily refer to a WS.sub.2 monolayer, although it might. In many embodiments of this invention, a layer of any material means a two dimensional crystal of that material.

Preparation of Two-Dimensional Crystals and Two-Dimensional Heterostructures

Large Scale Production of Graphene

[0128] There are many ways of producing graphene. These include CVD, liquid exfoliation, mechanical cleavage and all of these methods are suitable for the production of photovoltaic cells and devices according to this invention (see examples). The following is a continuous method by which large sheets of monolayer graphene can be produced.

[0129] The fact that this roll-to-roll method is a continuous process allows the industrial production of graphene on a scale suitable for the production of the photovoltaic devices and cells described in this application. There are three essential steps in this roll-to-roll process. [0130] 1. Adhesion of an adhesive scaffold to graphene after growth on copper. [0131] 2. Removal of copper (leaving graphene attached to the tape above). [0132] 3. Transfer of graphene to another substrate (usually a transparent polymer film such as PET to form a conductive coating) and removal of the carrying tape layer.

[0133] The growth process is as follows: [0134] 1. A quartz tube is used as a reactor furnace for the growth. A 30 inch long copper film can be inserted into the furnace. [0135] 2. The copper is annealed in H.sub.2 gas flow at 1000 C. to increase copper grain size. [0136] 3. Growth of monolayer graphene then occurs at a temperature of 1000 C. when a mixture of H.sub.2 and CH.sub.4 (methane) is allowed into the chamber followed by rapid cooling under H.sub.2 flow. [0137] 4. Thermal release tape is then applied to the surface of the graphene by passing the tape/graphene/copper film between two rollers. [0138] 5. This combined system is then passed through a bath of copper etchant and rinsed to remove the copper film.

[0139] Finally, the graphene/tape film is passed-along with the target substrate-through another set of rollers while being exposed to a temperature of around 100 C. to cause the tape to be released from the graphene surface, leaving the final product.

[0140] It is possible to dope graphene in the same process.

Preparation of Other Two-Dimensional Crystals

[0141] Two-dimensional materials can be made in bulk using the following technique for the exfoliation of a large number of layered crystals. The crystals come in commercially available powder form with flakes 1-10 microns in size. The crystals are placed in one of various solvents in a phial and placed in an ultrasonic bath. The 20 best solvents for WS.sub.2, MoS.sub.2 and BN are listed in Table I below. The ultrasound passing through the water breaks up the crystals so the layers separate and become dispersed in the solvent. Their lateral size in also reduced as it is quite aggressive procedure. The resultant flakes have a flake size of between 10 nm and 1 micron depending on the material. This mixture can then be subjected to centrifugation.

TABLE-US-00001 TABLE 1 (Taken from: J. Coleman, Two-dimensional Nanosheets Produced by Liquid Exfoliation of Layered materials Science, vol 331, no 6017, pp 568-571, 2011) BN MoS.sub.2 WS.sub.2 A/1 A/1 A/1 (AU) (AU) (AU) Solvent 300 nm Solvent 670 nm Solvent 630 nm 1 Cyclohexyl- 100 NVP 100 DMSO 100 pyrrolidinone (CHP) 2 N-dodecyl- 61 N8P 98 NVP 92 pyrrolidone (N12P) 3 Benzyl Benzoate 44 N12P 97 NMF 90 4 Isopropanol 44 CHP 88 N12P 84 5 N-Octyl- 44 NMP 80 DMEU 75 pyrrolidone (N8P) 6 N-vinyl 41 DMEU 73 DMF 73 Pyrrolidinone (NVP) 7 Benzyl Ether 40 DMSO 61 Benzyl 71 benzoate 8 Dimethyl- 36 DMF 54 CHP 69 imidazolidinone (DMEU) 9 Cyclohexanone 29 DMA 54 Cyclohexanone 63 10 Chlorobenzene 28 Benzaldehyde 51 Benzonitrile 59 11 Dimethylsulphoxide 27 Benzonitrile 47 N8P 59 (DMSO) 12 Benzonitrile 26 Benzyl 46 Isopropanol 59 benzoate 13 Chlorobenzene 25 NMF 41 DMA 57 14 Chloroform 23 Cyclohexanone 38 Benzylether 55 15 Bromobenzene 23 Isopropanol 32 Chlorobenzene 45 16 N-methyl- 23 Quinoline 26 Methanol 45 pyrrolidinone (NMP) 17 N-Methyl 21 Acetone 24 Formamide 40 Formamide (NMF) 18 Dimethylformamide 18 Benzylether 23 Bromobenzene 29 (DMF) 19 Dimethylacetamide 16 Cyclohexane 22 Quinoline 26 (DMA) 20 Formamide 9 Methanol 21 Acetone 17

[0142] The centrifugation process involves spinning a phial of the dispersion at a high rpm in order to separate the large flakes from the smaller ones. The result is a dispersion with a gradient in the concentration with the thick large heavy flakes at the bottom. The low concentration dispersion with the small flakes can be removed from the top using a pipette or other method.

[0143] If a paper is wanted, this solution can be filtered to remove a laminate of the flakes assembled in a roughly planar structure.

[0144] An alternative intercalation process can be used for the preparation of TaS.sub.2 two-dimensional crystals. The crystals are grown by vapour transport. This process involves placing tantalum and sulphur in a quartz tube with a transport gas such as iodine which carries the constituents down a temperature gradient and allows deposition of the resultant crystal. This is usually done at temperatures of around 800-1000 C.

[0145] The crystals are removed from the growth chamber and exposed to n-butyl lithium in a water free environment. The n-butyl lithium penetrates between the layers and subsequent exposure to water leads to a reaction between Li and water. This reaction releases hydrogen gas that pushes the layers apart. A suspension of thin flakes is left in the water which can then be drop cast onto a desired substrate.

Preparation of Two-Dimensional Heterostructures

[0146] A clean and precise technique for the manual transfer of thin crystal flakes to the surfaces of one another is as follows. This method can be used in the preparation of complex stacks of multiple layers and also in the preparation of complex devices. The technique is generally applicable but is described below in connection to hBN/graphene heterostructures.

[0147] The technique involves various steps as follows: [0148] 1. Single crystal hBN flakes were deposited on an oxidised silicon wafer using the scotch tape technique, i.e. bulk crystals are peeled many times with adhesive tape and pressed onto the oxidised silicon substrate. Some proportion of the resultant debris will be single crystal hBN flakes with a thickness on the order of 30-50 nm, a suitable thickness substrate to eliminate roughness from SiO.sub.2. [0149] 2. Separately, a bilayer polymer stack is spin coated onto another substrate. A graphene flake is depositedby the same method as described above-onto this polymer stack. The two different polymers are not soluble in the same solvent. This way the bottom one can be selectively dissolved (by water in this case) without affecting the top layer which is carrying the graphene. The top layer-now floating on top of the solventcan be picked up on a glass slide, inverted and positioned about the hBN flake under an optical microscope. The two are carefully brought into contact. The sacrificial top polymer layer is then dissolved in another solvent to leave the hBN/graphene stack.

[0150] The stack is made of a water soluble polymer layer and PMMA which can be dissolved in acetone.

Further Discussion of the Invention

[0151] This invention generally relates to new applications of graphene. Specifically, the invention relates to new graphene heterostructures, applications of graphene heterostructures and methods of making graphene heterostructures.

[0152] It is an aim of the invention to provide methods of making graphene heterocycles which are more energy efficient than existing methods. The methods may be quicker than existing methods. They may generate less waste than existing methods.

[0153] It is an aim of this invention to provide methods which allow access to graphene heterostructures which it is not possible to make using existing methods.

[0154] It is an aim of this invention to provide methods which allow the efficient production of graphene heterostructures on a larger scale than existing methods.

[0155] A further aim of the invention is the provision of new graphene heterostructures. These heterostructures may have similar properties to existing graphene heterostructures but be easier to produce. The new graphene heterostructures may have improved properties compared to known graphene heterostructures, or they may have improved properties compared to known non-graphene based materials. The graphene heterostructures may have new properties not previously observed in graphene heterostructures. In particular, the new heterostructures may have new combinations of properties not previously observed in a single material, whether that material is graphene based or not graphene based.

[0156] Another aim of the invention is to provide heterostructures for use in new photonics devices, such as LEDs and photovoltaic devices. The heterostructures may be for use in new electronic devices, such as transistors.

[0157] Further aims of the invention will be made clear in the following sections:

Introduction to the Further Discussion of the Invention

[0158] Technological progress is determined, to a great extent, by developments in material science. The most surprising breakthroughs are attained when a new type of material, or new combinations of known materials, with different dimensionality and functionality, are created. Well-known examples are the transition from three-dimensional (3D) semiconducting structures based on Ge and Si to 2D semiconducting heterostructures, nowadays the leading platform for microelectronics. Other examples are quantum wells formed at GaAs/AlGaAs semiconductor interfaces for high electron mobility transistors, magnetic multilayers for hard discs, and solar cells based on 2D organic LEDs. Ultimately, the limits and boundaries of certain applications are given by the very properties of the materials naturally available to us. Thus, the band-gap of Si dictates the voltages used in computers, and the Young's modulus of steel determines the size of the construction beams.

[0159] Here we describe a new paradigm in material science, physics, engineering and nanotechnology: heterostructures based on 2-dimensional crystals that will decouple the performance of particular devices from the properties of naturally available materials. The ultimate goal is to develop a new paradigm of materials on demand with properties precisely tailored for novel complex architectures and structures.

[0160] Our general approach is based on our combined know-how and unique expertise in the new class of 2D atomic crystals, such as graphene, graphane, flurographene, monolayers of BN and MoS.sub.2, etc. Individual 2D crystals can be arranged into heterostructures, thus creating stacks with novel properties, very different from those of individual components. It is also possible to intercalate metal nanoparticles, nanotubes, semiconductor quantum dots (QD) and wires, to create novel 1 D-2D compounds.

[0161] Heterostructures have already played a crucial role in technology, giving us semiconductor lasers and high-mobility field effect transistors (FET). However, thus far the choice of materials has been limited to those which can be grown (typically by molecular beam epitaxy) one on top of another, thus limiting the types of structures which can be prepared. Instead, 2D crystals of very different nature can be combined in one stack with atomic precision, offering unprecedented control on the properties and functionalities of the resulting 2D-based heterostructures. One can create such layered heterostructures by placing individual 2D crystals mechanically, epitaxially, from solutions. 2D materials with very different properties can be combined in one 3D structure, producing novel, multi-functional materials. An alternative class of nanostructures will also be investigated by combining metal or semiconductor nanoparticles (eg gold, Nanodiamond, CdSe, PbSe, PbS, etc) with the 2D superstructures. Solution processing, self-assembly, intercalation and lithography will allow an entire new class of hybrid structures.

[0162] Most importantly, the functionality of such heterostructures is not simply given by the combined properties of the individual layers. Interactions and transport between the layers allow one to go beyond simple incremental improvements in performance and create a truly quantum leap in functionality.

[0163] These 20 heterostructures can change the whole paradigm of material science. By combining materials with very different properties, we are no longer limited by a single set of parameters. The resulting 3D structures will combine conductivity of one 20 crystal, strength of another, chemical reactivity of the third and the optical properties will be determined by the whole heterostructure. By carefully choosing and arranging the individual components one can tune the parameters, creating materials with tailored properties, or materials on demand. Following this novel approach, part of the functionality is brought to the level of the design of the material itself (usually it is done only when creating a structure from a given material).

Background to the Further Discussion of the Invention

[0164] The class of 2D atomic crystals started with graphenea monolayer of carbon atoms arranged into a hexagonal lattice. It is a remarkable material with myriads of unique properties, from electronic to chemical and from optical to mechanical. It has also opened a floodgate for many other 2D crystals to be discovered and studied. Such crystals are stable, mechanically strong and carry many properties which cannot be found in their 3D counterparts. Our research has also shown that such two-dimensional crystals have an unexpectedly high electronic quality. This has generated a flood of further experimental discoveries. The research subject remains one of the most active within the whole areas of physics, materials science and nanotechnology. Among nearly a dozen of atomically thin materials that were demonstrated so far, graphene stands out due to its unique electronic spectrum and ballistic transport on a micron scale under ambient conditions. It is the strongest material available to us, its conductivity is millions higher than copper, it has very high thermal conductivity, etc. The linear dispersion of the Dirac electrons combined with the strong and peculiar interaction with light, makes graphene ideal for photonics and optoelectronics. The fact that graphene is an ultra-thin material plays an important role allowing the ambipolar electric field effect, and unrivalled electrostatics for scaling of electronic devices to nm sizes.

Applications are no longer wishful thinking, and graphene can now be produced in industrial quantities. Within the short 7 years period, this research has moved from academic to industrial labs, which will continue to support strong interest in graphene for years to come. The other 20 crystals carry a range of interesting complementary properties. BN is an insulator with a large band gap (6 eV) and even a single layer creates a tunnelling barrier with resistance of about lk-m.sup.2. It is sometimes called insulating graphite and it might be used in instances where graphene's high conductivity is a disadvantage (ultra-thin, high quality, insulating layers for nano-electronics, non-conductive, ultra-strong, composite materials). MoS.sub.2 is a semi-conductor and provides a smaller tunnelling barrier, its band-gap is ideal for optoelectronics. NbSe.sub.2 is a superconductor, Bi.sub.2Te.sub.3-topological insulator, etc.

Mechanical Exfoliation of Layered 3D Crystals

[0165] This is the most quick & easy, as well as efficient method for 2D crystals production. Several materials (graphene, monolayers of BN, MoS.sub.2) have already been created by this method, and those usually demonstrate very high electronic and crystallographic quality. The properties of the 2D crystals are typically very different from those of their 3D precursors; graphite is semimetal, while the overlapping between the valence and the conduction bands in mono- and bi-layer graphene is exactly zero. Bulk MoS.sub.2 is an indirect-gap semiconductor, while its monolayer has a direct band gap. However, there is still a large number of other layered materials which potentially can be cleaved and prepared as monolayers. Most interesting are Bi.sub.2Te.sub.3 as topological insulator, NbSe.sub.2 and MgB.sub.2 as superconductor and many others. Furthermore, often several layers of such materials would possess properties which are intermediate (or even completely different) between monolayer and bulk. A broad range of techniques (transport, TEM, AFM, Raman, optic) can be used for characterisation of the new 2D crystals.

Liquid Phase Exfoliation (LPE)

[0166] A mass-scalable approach to 2D crystals is to exfoliate their bulk counterparts via chemical wet dispersion followed by ultrasonication, both in aqueous and non-aqueous solvents. This technique offers many advantages for cost reduction and scalability. The lateral size of the layers can be controlled from few nanometers to microns. The number of layers can also be controlled via separation in centrifugal fields or by combination with density gradient ultracentrifugation (DGU). The availability of solutions and dispersions opens up a range of applications in composites, thin films and inks. Inks can be printed in a variety of ways, and mixed to create hybrids. Many applications in photonics and optoelectronics, such as transparent conductors, third generation solar cell electrodes, and optical-grade polymer composites will benefit from LPE produced and assembled materials. LPE can also produce ribbons with widths<10 nm, allowing a further in-plane confinement of the 2D materials, thus an extra handle to tailor their properties. LPE does not require transfer techniques and the resulting material can be deposited on different substrates (rigid and flexible) following different strategies such as dip and drop casting, spin, spray and rod coating, ink-jet printing, etc. Several layered materials (including MoS.sub.2, WS.sub.2, MoSe.sub.2, MoTe.sub.2 TaSe.sub.2, NbSe.sub.2, NiTe.sub.2, BN, and Bi.sub.2Te.sub.3) have been successfully exfoliated using this simple, yet efficient method. This approach may also influence the chemical stability, as the layer of liquid might protect those crystallites from oxidation. Such suspensions allow easy assembling of the materials into superstructures.

Chemical Modification of Existing 2D Crystals

[0167] This is a powerful method for the synthesis of 2-D crystals. As an example graphene can be considered as a giant molecule, which can be modified through chemical reactions. Several stable graphene derivatives have been already produced including graphane, fluorinated and chlorinated graphene. Clearly it is only the beginning and many other materials with very different properties are possible. It is expected that other 2D materials are similarly versatile and allow as many chemical modifications as graphene does. Separate chemical modification of the two sides of a 20 crystal allows an electric moment can be created across the crystal and lead to new, unexpected properties. Also, chemical modifications can be used as a tool to provide a fine control over the distance between neighbouring planes in the 2D heterostructures. This will also be done by intercalation or by placing other metallic and semiconducting nanostructures between the planes.

Mechanical Transfer

[0168] Transfer of individual 2D crystals into heterostructures has been developed and has already led to the observation of several interesting effects, including fractional quantum Hall effect, ballistic transport and metal-insulator transition in graphene. An additional advantage is the possibility to control/modify each individual layer as it is being deposited. Such chemical modification is possible at any stage of the transfer procedure. Also, any atomic layer in the stack can be individually contacted, offering unprecedented control on the properties of the stack (effectively we have a material with contacts to every individual atomic plane). It has been demonstrated that local strain can significantly modify the band structure of graphene and other 2D crystals. Thus, it is possible to apply strain, locally produced by AFM tip, for additional control on the electronic and optical properties of the stacks. Also important is the control of the relative orientation of the layers. Realistic control of 0.2 rotation between them would allow fine control over the electronic properties of the stack.

Heterostructures Deposited from Suspensions

[0169] Large scale placement of LPE samples can be achieved by spin coating and Langmuir-Blogdett. Surface modifications by self-assembled monolayers enable targeted large-scale deposition. High uniformity and well defined structures on flexible substrates can also be obtained. Di-electrophoresis can also be used to control the placement of individual crystals between pre-patterned electrodes. Inkjet printing allows to mix and print layers of different materials and is a quick and effective way of mass-production of such systems. Although the quality of the resulting structures can be significantly lower than that obtained by mechanical or CVD methods, it is still be suitable for a number of photonics and optoelectronics applications, as well as for applications in thin film transistors, RF tags, solar cells, batteries and supercapacitors.

CVD Growth

[0170] CVD is in principle the most powerful method for mass production of the heterostructures. Graphene and graphene layers can now be grown on various substrates (Ni, Cu, Ir, Ru, etc) when the latter are exposed to hydrocarbon gases, such as benzene, ethane, and methane, with a suitable reaction temperature. Reduction of growth temperature is desirable in order to cut production costs, and directly integrate with CMOS processing. Plasma-enhanced CVD can be used to lower growth temperature. Unsupported flakes can also be produced. A major factor impacting large-scale production of graphene-based nano-electronic devices is access to high-quality graphene layers on insulating substrates. There are several indications that the growth of other 2-D materials is indeed feasible. Hexagonal boron nitride (h-BN) has already been shown to be effective as a substrate for graphene CVD. In fact. CVD graphene on h-BN has shown remarkable mobilities, much higher than for graphene grown on metal substrates.

[0171] MBE is an Ultra-High-Vacuum-based growth technique for producing high quality epitaxial structures with monolayer control. Since its introduction in the 1970s as a tool for growing high-purity semiconductor films, MBE has evolved into one of the most widely used techniques for epitaxial layers of metals, insulators and superconductors, both at the research and the industrial level. MBE was demonstrated an effective tool to grow carbon films directly on Si(111) and is a promising approach to achieve high-purity graphene heterostructures on a variety of substrates such as SiC, Al.sub.2O.sub.3, Mica, SiO.sub.2, Ni, etc. MBE is also quite promising for in situ growth of hetero- and hybrid structures, combining graphene and semiconductors.

Electronic Transport in Lateral and Vertical Structures

[0172] Vertical and lateral transistors are the first and most natural application of atomically thin heterostructures and multilayer systems. Vertical heterostructures and tunnel devices have been used for many years, from the Esaki diode to cascade lasers. 2D-based heterostructures offer a unique prospect of extending the existing technologies to their ultimate limit of using monolayer-thick tunnel barriers and quantum wells. At the same time, since the doping-dependent screening properties of graphene can be controlled electrically, graphene sheets and thin ribbons in multilayer structures can be used as gates with widely variable propertiesa functionality hardly offered by any other material. We believe that heterostructures, built by one of the methods listed above, offer unique opportunities to study transport properties of complex, interacting systems (for example, exciton condensation) and to use such structures for transistor with significantly improved transfer characteristics, sensors and other applications. Vertical devices can also be scaled to one nm laterally, as far as lithography techniques allow.

Tunnelling Devices and Resonant Tunnelling

[0173] Tunnelling devices based on 2D heterostructures have been demonstrated, showing that BN can act as an excellent defect-free tunnel barrier. The BN monolayer separating two graphene electrodes provides a high-quality tunnel barrier and allows biases as large as 1V without electrical breakdown. The first experiments on exfoliated Gr/BN/Gr structures (Gr stands for graphene) demonstrate a complex non-linear tunnelling I-V.

[0174] There are various types of devices where quantum tunnelling us used (for example, tunnelling magnetoresistance devices or resonant tunnelling diodes). Automatically thin, smooth and continuous barriers offered by the use of 2D crystals can dramatically improve quality and characteristics of any existing or considered scheme involving quantum tunnelling.

Photovoltaic Devices

[0175] We will create superstructures composed of two metallic plates separated by a barrier with modulated profile. Type-I quantum wells can be formed and injection of electrons and holes with subsequent recombination will lead to light emission. Using more complicated structures, both type-I and type-II quantum wells of various configurations can be created. As the band-structure of 2D materials depends on the number of layers, simply by changing the thickness of one of the components we could strongly tune the resulting optical properties.

[0176] Photovoltaic devices can be created by placing two metallic 2D crystals (for instance graphene) within tunnelling proximity of each other. An example would be two graphene layers, separated by several layers of BN, which serves as a tunnelling barrier. Electric field separates the electron hole pair which is created by an incoming photon, resulting in photocurrent. By applying a bias voltage (or by using proximity effect of other metallic 2D crystals) electric field will be created inside the barrier. Any e-h pair excited by light is separated and contribute to photocurrent. It is possible to create heterostructures with various band gaps, sensitive to photons of different energies.

Active Plasmonic

[0177] Combinations of 2D heterostructures with plasmonics allows for the creation of active optical elements. 2D heterostructure are ideally suited to be used with plasmonic structures, as they can be positioned exactly at the maximum of electric field from plasmonic nanostructures. Such elements are of great importance in different areas of science and technology: from ubiquitous displays, to high tech frequency modulators. Despite great progress in optical disciplines, active optics still relies heavily on either liquid crystals, which guarantee deep modulation in inexpensive and small cells, but are quite slow, or non-linear optical crystals, which are fast, but bulky and expensive. Thus, the development of inexpensive, fast and small active optical elements would be of considerable interest.

[0178] Plasmonic metamaterials of various configurations can be sandwiched between Gr/BN heterostructures (e.g. golden dots can be placed between two graphene/BN sandwiches). The conductivity of graphene can be changed several orders of magnitude by electrostatic doping. Such change could modulate the optical properties of the underlying plasmonic structure. The combination of 2D heterostructures with plasmonics could result in fast, cheap and small active optical elements.

[0179] Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0180] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0181] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Example 1Preparation of and Photovoltaic Ability of a Graphene/WS.SUB.2./Graphene Device

[0182] The experiments described in the following sections demonstrate a beneficial combination of the properties of graphene and WS.sub.2: WS.sub.2 as a good photoactive material and graphene as a good transparent electrode. Using a graphene/WS.sub.2/graphene stack with appropriately positioned Fermi levels and simply doping the two graphene layers differently (either by electrostatic gating or chemical methods), leads to large photocurrent. The layered nature of our structures means it is possible to fabricate large area light harvesting devices. Without illumination, such devices act as tunnelling transistors.

Devices comprising three principal layers, top and bottom graphene electrode layers (we tested both micromechanically cleaved and CVD-grown graphene) sandwiching a photoactive WS.sub.2 layer, were prepared. The left hand panel in FIG. 1 shows an optical image of a device according to the invention. In the fabrication procedure, the flakes were transferred with the so called dry transfer technique (in the case of micromechanically cleaved graphene) with thorough annealing (H.sub.2/Ar (10:90) at 200 C. for 3 hrs) at each stage to ensure minimal contamination between the layers and low level doping of the graphene layers. Hexagonal boron nitride (hBN) was chosen as both a substrate and an encapsulating layer to achieve a higher doping homogeneity and spacer to a carefully define the contact region. Thus, the final structure of a typical device on top of the oxidised silicon wafer was graphene/hBN/WS.sub.2/graphene/hBN. Here, the SiO.sub.2/hBN could be used as the gate dielectric. A series of such structures was produced where the thickness of the WS.sub.2 layer was varied from 5-50 nm. Devices with WS.sub.2 thickness of 20 nm were chosen for the devices presented here because they were found to absorb a fair proportion of incident photons and still allowfull characterisation by tunnelling spectroscopy.
A stepwise summary of an exemplary process for preparing the heterostructures, cells and devices according to the invention is as follows: [0183] 1. The bottom graphene flake is mechanically exfoliated onto silicon oxide. (380 m doped n-type doped silicon with a dry thermal 300 nm oxide with a polished finish. The process of exfoliation is performed by repeatedly peeling layers of the parent crystal with adhesive tape and firmly pressing the debris onto the wafer's surface. A suitably thin flake is found using an optical microscope. [0184] 2. In order to transfer further flakes a transfer procedure is used, which is as follows. [0185] a. A substrate (can be another silicon wafer) has PMGI (Poly(methyl glutarimide)) spin-coated on one surface with a thickness of 200 nm. The PMGI is then baked on a hot plate at 140 C. [0186] b. A layer of PMMA (Poly(methyl methacrylate)) is then spin-coated on the surface of the PMGI with a thickness of 400 nm and again baked on a hot plate at 140 C., see FIG. 1. [0187] c. TMDC flakes are exfoliated in the same way as described in step 1, directly onto this polymer stack. [0188] d. After finding an appropriate flake with an optical microscope, the PMGI layer is dissolved with MF319, leaving the PMMA/flake floating on top of the solvent. [0189] e. A metal washer is used to fish the membrane from the solvent. The structure is inverted and positioned above the target substrate using a modified mask aligner. [0190] f. The two are carefully brought into contact, after which the PMMA is dissolved in acetone. [0191] g. The structure can then be annealed in up to 250 C. in a H.sub.2/Ar 10:90 mixture. [0192] 3. A further graphene flake is transferred onto the stack in the same way as described in step. 2. [0193] 4. Using either optical or e-beam lithography and metal deposition, electrodes contacted to the device; one electrode to each graphene layer, see FIG. 2.
The IV characteristics of our samples strongly depend on illumination, see FIG. 5a. Without illumination, the devices displayed strongly non-linear IV curves (FIG. 5a inset). Comparing this with the main figure, one can see that this was in strong contrast to when they were illuminated: the resistance dropped by more than 3 orders of magnitude and the curves became linear around zero bias. At higher bias (0.2 V) they began to saturate, as the number of available charge carriers in the photoactive region was limited.
To elucidate further, the photocurrent generated in our devices was mapped by scanning photocurrent microscopy, where a laser spot was scanned over the sample, and the resultant photocurrent displayed as a function of laser spot position. In FIG. 4c one can see that a current is generated only in the region where all three principal layers overlap. With reference to FIG. 5b, we explain the origin of the photocurrent by examining the collective band diagram. In the ideal case the structure is symmetric (FIG. 5b top) and the electrons/holes generated in WS.sub.2 (by absorption of a photon with sufficient energy) will have no preferred diffusion direction and hence no net photocurrent is measured. However, in the presence of a built-in electric field (FIG. 5b bottom) across the WS.sub.2 (either due to a difference in the initial doping between the graphene sheets or by gating), the e-h pairs will be separated and a photocurrent measured.
The effect was investigated by taking photocurrent maps at different gate voltages V.sub.g, see FIG. 4c. Immediately after fabrication (which involves the annealing stage), the devices showed a minimum in the integrated photocurrent close to zero gate voltage (FIG. 1c, upper panels, the same can be seen in FIG. 5a). For any finite V.sub.g (either positive or negative) the photocurrent increased proportionally to V.sub.g but began to saturate at 20 V, again due to the finite number of generated charge carriers. After leaving a non-encapsulated sample in a humid atmosphere for 1 day the photocurrent maps were retaken (FIG. 4c, bottom panels). The photocurrent at zero gate voltage became finite (positive) and the response with gate voltage was shifted by 20 V. The effect is also seen in FIG. 5a where the intercept of the IV curves is shifted due to movement of the chemical potential in graphene. In general our devices also showed a strong gate dependence without illumination, which allows them to be used as tunnelling transistors. The highest measured ON/OFF ratio (highest to lowest current modulation) for our devices was 10.sup.6.
The photocurrent observed in these devices is surprisingly strong for only a few atomic layers of TMDC, but this strong light-matter interaction can be understood from the nature of the electronic states in this material. Ab initio calculations were performed for the DOS and the joint density of states, JDOS, of three single layer semiconducting TMDC: WS.sub.2, WSe.sub.2, and MoS.sub.2. It is clear that in the visible range there are strong peaks, associated with van Hove singularities, in the DOS that lead to enhanced light absorption, and importantly this is a feature that is universal to TMDCs. These van Hove singularities come from the nature of the electronic wave functions: while the valence band is essentially composed of states coming from the d orbitals of the transition metal (TM), the conduction band is characterized by a linear superposition of d orbitals of the TM and p orbitals of the chalcogen atoms. The p orbitals have a localized nature and are also responsible for the a bands which in turn are responsible for the structural stability of these materials (analogous to what happens in graphene). The localized character of the electronic bands (that is, the large effective mass of the carriers) leads to the peaks, i.e., van Hove singularities, in the DOS which are responsible for the enhanced photo-responsivity of these materials from the nanoscopic down to atomic scale. A direct measure of the effect of the van Hove singularities in the optical response of TMDC is given by the JDOS, defined as:

[00001]$\mathrm{JDOS}\left(E\right)=\frac{1}{4.Math..Math.{}^2}.Math.d^2.Math.k.Math..Math.\left(E_{v,k}-E_{c,k}-E\right)$

where V and C are the valence and conduction bands, the JDOS is a direct measure of the so-called joint critical points, that is, the van Hove singularities in the Brillouin zone around which a photon of energy, hE.sub.cE.sub.v, is very effective in inducing electronic transitions over a relatively large region in momentum space. The large contribution to the transition probability for joint critical points gives rise to the structure observed in the frequency dependence of the optical properties of the TMDC. Thus, the photocurrent, I(), at some light frequency is proportional to JDOS(h). There is a sharp rise in the photo-absorption in the JDOS(E) in the visible range of all TMDC studied. In order to further confirm that our results are not dependent on the thickness of the TMDC, we have shown that the DOS and JDOS for bulk (3D) semiconducting TMDCs. The presence of sharp peaks in the DOS and the sharp rise of the JDOS is comparable with the values found for a single layer. Hence, the strong light-mater interactions in semiconducting TMDCs is not a unique feature of the bulk material and it can be extended to monolayers.
The effect discussed here has a similar, albeit with a different physical origin, to the strong Raman absorption in 1D semiconducting carbon nanotubes. In that case, the 1D nature of the material leads to 1/{square root over (E)} singularities in the DOS at the top (bottom) of the valence (conduction) bands, leading also to strong light-matter response.
We have also computed the work function, , for the semiconducting TMDCs studied here. We find that the work functions vary considerably depending on the transition metal used (for monolayer, .sub.WS.sub.22 eV, .sub.WSe.sub.21.45 eV, .sub.MoS.sub.22.6 eV) and their thickness (for bulk, .sub.WS.sub.21.5 eV, .sub.WSe.sub.21.2 eV, .sub.MoS.sub.21.98 eV). Notice that since we are using graphene as an transparent electrode (.sub.G4.5 eV), these differences in work function have a direct impact in the device performance.
We also investigated the performance of our prototype light harvesting devices. For power generation, an important parameter is the extrinsic quantum efficiency (EQE), defined as the ratio of the number of charge carriers generated to the number of incident photons. This can be expressed in terms of the photocurrent I, incident power per unit area P and excitation wavelength by

EQE=((h*c)/(e*))*(I/P)

where h is the Planck constant, c the speed of light in vacuum and e the electron charge. FIG. 3 shows the effect on the IV characteristics, of irradiance with different intensities. Again, all measured IV curves were linear in the low bias regime and with slopes dependent on the illumination intensity. Using the relation for EQE we calculate the efficiency, shown in FIG. 7, where the data were collected for several wavelengths at zero bias and V.sub.g=40 V. The extrinsic quantum efficiency did not appear to be dependent on wavelength, as expected from the approximately constant optical absorption, over this range. It is likely that the decrease in quantum efficiency with increasing power is due to screening of the built-in electric field by the excited electrons in the conduction band of WS.sub.2. In real devices it is likely that either a thicker WS.sub.2 layer be used or a multiple structures stacked upon each other in order that all incident light is absorbed within the device, pushing EQE even higher.

Conclusions

[0194] Here we have shown that the fabrication of graphene-WS.sub.2 hybrid devices allows the production of prototypical efficient solar cells. We are able to reach an extrinsic quantum efficiency of 30% which is expected to be higher under lower intensity illumination (e.g. solar radiation) and by optimizing the device design. The photocurrent generated was seen to be almost independent of wavelength from 488 to 633 nm. The same device can be used as a transistor which has ON/OFF ratios exceeding 10. The use of various TMDC as well as their combinations would allow one to create photovoltaic devices with sensitivity in the predetermined spectral range, suitable for large scale production. Devices made from micromechanically cleaved and CVD graphene demonstrate very similar behaviour.

Methods

[0195] Graphene and thin graphite flakes were produced by micromechanical exfoliation of graphite (graphite source). We used single crystal WS.sub.2 supplied in powder form by Sigma-Aldrich. Despite an average crystal size of only 2 m it is possible to find crystals up to 50 m that could be exfoliated as well.

[0196] Recent progress has led to relatively facile fabrication of graphene hybrid devices with a large degree of versatility. The method allows flakes of layered materials to be transferred to the surfaces of one another with a high degree of accuracy and cleanliness. In this way stacks of different materials can be created with precise control over the constituents of the new hybrid material.

[0197] We have used a so-called dry transfer technique to create these structures. This technique involves the mechanical exfoliation of the required flakes onto a dual layer polymer stack (PMGI+PMMA). The bottom polymer (PMGI) layer can be selectively dissolved and the resulting membrane inverted and positioned above the target flakethe initial bottom flake was instead cleaved onto a Si/SiO.sub.2 wafer (290 nm oxide). After each transfer the top polymer layer (PMMA) was dissolved and annealed thoroughly in a gaseous mixture of H.sub.2/Ar (10:90) at 300 C. before the subsequent transfer of the next flake. In this way stacks with an arbitrary number of layers can be produced. Once the required flake stack had been fabricated, electrical contact was made via standard photolithographic processing and e-beam evaporation of a Cr adhesion layer (5 nm) and Au (50 nm) and placed in a package for measurements. In order to make scanning photocurrent microscopy measurements we utilize a WiTEC scanning Raman setup. The sample was placed onto a piezoelectric stage with laser light incident from above. The laser was focussed by a 100 microscope objective with a laser spot size is diffraction limited (diameter500 nm). The laser spot is scanned over the surface and the resultant current flow between the two graphene electrodes is measured alongside the conventional Raman spectra for each point in the scan

We used chemical vapour deposition (CVD) to fabricate high-quality, large area graphene electrodes. The graphene was grown on 25 m thick copper (Cu) foil (from Alfa Aesar, item no. 13382). Before graphene deposition, the Cu foils were cleaned with subsequent washes in acetone, DI water and IPA in order to remove both organic and inorganic contamination from the surface. To further improve the CVD graphene quality and increase grain size, the Cu foil was then annealed in a quartz tube for 30 minutes at 1000 C. in a flux of H.sub.2 at 20 sccm and 20 mTorr. Graphene was grown on the Cu surface by adding 40 sccm CH.sub.4 to the gas flow (chamber pressure 600 mTorr) whilst maintaining a temperature of 1000 C. The sample was allowed to cool in a H.sub.2 atmosphere and then removed from the chamber at room temperature. The graphene could then be transferred to a silicon wafer by etching of the Cu foil.

Device Structure and Photocurrent Mapping

[0198] Referring to FIG. 4: The top panel shows an optical micrograph of one of our devices and a schematic cartoon of the device. The shading of the three constituent layers denotes the regions of the respective materialstop and bottom graphene electrodes shown in red and blue, while WS.sub.2 is shown in green. The bottom panel shows photocurrent maps taken before (above) and after (below) the device was stored in a humid atmosphere to change doping. The signal is only seen in the area where all three layers overlap. The gate voltage at which the total signal switched sign moved 20 V after the device was stored in a humid atmosphere for 1 day.

Gate Dependent IV Characteristics

[0199] FIG. 5 shows: IV curves taken at gate voltage values from 20 to +20 V in 10 V steps, as signified by the arrow. The laser illumination energy was 2.54 eV and the power 10 W. The curves are linear at low bias but saturate at higher bias due to limited available charge carriers. The inset shows the transistor behaviour of the same device over a larger gate voltage range. This particular device exhibited an ON/OFF ratio510.sup.3.

Photoresistivity

[0200] FIG. 6. shows IV characteristics of the device shown under laser illumination of varying intensity. An external bias is applied between the graphene electrodes and the resultant current flow is measured. The resistivity of the device changes only when laser light is incident on the region where all three constituent flakes overlap. Shown are IV curves taken with a 2.54 eV laser set to a total power of 10, 20 and 30 W and at 20 V.sub.g. The inset shows an IV taken without illumination at 20 V.sub.g, the current undergoes a large modulation when the device is positively biased.

Quantum Efficiency

[0201] Referring to FIG. 7: the external quantum efficiency of the devices is the ratio of the number of charge carriers to incident photons. Due to the small variation in optical absorption across this wavelength range the data for different wavelengths collapse onto a single curve. The inset shows (as open squares) the photocurrent measured with a 1.95 eV laser as a function of intensity and follows a sublinear dependence. This results in the largest quantum efficiency values at low intensities.

Example 2Preparation of a Device of the Invention Using Solution Processed Materials

[0202] An alternative exemplary method of preparing the devices, cells and heterostructures according to the invention is as follows. [0203] 1. Metal electrodes (Cr/Au (5/50 nm) in our case) are patterned onto a substrate (in this case silicon/silicon dioxide). [0204] 2. A WS.sub.2 film is prepared as follows. [0205] a. WS.sub.2 powder is put into a 35% ethanol/water mixture and placed in an ultrasonic bath for 5 days to break up the WS.sub.2 crystals to few layer nanoplatlets which form a suspension. [0206] b. The suspension is filtered through a cellulose membrane and a film is left, attached to the filter. [0207] c. By dipping the membrane in water, a thin film (50 nm thick) delaminates from the WS.sub.2 film and is left floating on the water's surface. [0208] 3. The membrane can be fished from the water with the gold patterned substrate so that the WS.sub.2 film covers many metal electrodes. [0209] 4. CVD graphene can then be transferred as previously described to form the top, transparent electrode. [0210] 5. At each overlap point of the metal/WS.sub.2 film/graphene a device is formed. The size of the devices are limited by the size of the substrate and cellulose membrane not by the active materials.

Example 3Preparation of a Flexible Device

[0211] A device as prepared in Example 1 was transferred to a PET (poly(ethylene terephthalate)) substrate and subjected to strain. As the device was put under strain, it's performance as a transistor was measured and it was found that there was no change of the current with a strain of up to about 5%. It was also shown that it is still possible to modulate the current when the device is under strain.

After it was subjected to strain, the photovoltaic properties of the device were tested as for the Si/SiO.sub.2 device described above.

[0212] The inset of FIG. 7 shows the photocurrent of the flexible device (inset, as crossed squares) after it has been placed under strain measured with a 1.95 eV laser as a function of intensity and follows a sublinear dependence.

[0213] In general, the EQE in our devices on Si/SiO.sub.2 substrate is somewhat higher in comparison with the flexible PET devices due to multiple reflections in SiO.sub.2 which effectively works as a cavity and increases the fraction of light adsorbed in WS.sub.2.

Example 4Preparation of a Graphene/MoS.SUB.2./Graphene Device

[0214] The results described above for WS.sub.2 devices apply universally to all the transition metal dichalcogenides. We have also shown that a similar behaviour was observed with MoS.sub.2. The devices were fabricated in the same fashion as described in Example 1. To summarise, the devices consist of a tri-layer structure comprising a TMDC flake sandwiched between two electrically isolated graphene layers which act as transparent electrodes. The device sits on an oxidised silicon wafer with the doped silicon acting as a gate electrode. The electric field across the semiconducting region can be altered by applying a voltage between the bottom graphene layer and the doped silicon back gate. The efficiency of our devices fabricated with MoS.sub.2 was found to be lower than for WS.sub.z. This is unexpected from the calculated DOS which are similar for all the TMDCs but it is speculated that a higher level of impurity atoms such as rhenium present in the MoS2 lattice could be responsible due to creation of impurity states in the band gap which increase the rate of recombination.

Example 5Preparation and Photovoltaic Ability of Devices Incorporating Gold Nanostructures

[0215] Two devices which incorporate a gold nanostructure were prepared:
A) A 1 nm thick gold film was thermally evaporated onto a pre-existing graphene/WS/graphene device which had previously been seen to exhibit a photovoltaic signal (the device of Example 1). The nanostructures in this case are self-forming as the gold (in the form of Cr/Au (5/50 nm)) does not make a continuous layer but instead forms islands. The signal was seen to increase by a factor of up to 15 following this procedure (FIG. 8).
B) A second device type was made (according to the method described in Example 1) in which the photoactive part of the device had two regions: one area where nanostructures were fabricated and one without. The structures are patterned using lithographic techniques. In this case the gold (in the form of Cr/Au (5/50 nm)) dots (disks) have a diameter of 150 nm and a pitch of 350 nm. The average photocurrent was greatly enhanced for the region with the gold dots (disks) compared to the region without the gold dots (FIG. 9)