Patent Publication Number: US-2023154747-A1

Title: A seed layer, a heterostructure comprising the seed layer and a method of forming a layer of material using the seed layer

Description:
TECHNICAL FIELD 
     The present disclosure relates to a seed layer, a heterostructure comprising the seed layer, and also a method of forming a layer of material using the seed layer. 
     BACKGROUND 
     Integration of heterogeneous materials is an important aspect for fabricating high performance semiconducting devices. For example, high speed and efficient optoelectronic devices such as light emitting diodes, infrared (IR) sensors, photodetectors and solar cells generally involve multilayer heterostructures which require integration of group III-V (GaAs, GaN, InP and others) and group II-VI (CdTe, CdS, ZnS, oxides and others) semiconductors with silicon (Si) microelectronics. The advantages of such integration are attributed to the superior optoelectronic properties achieved from the group III-V or the group II-VI materials, and the economic viability and compatibility of Si with complementary metal oxide semiconductor (CMOS) technology. 
     A conventional technique employed for forming such multilayer heterostructures is epitaxial growth, where an epilayer of material grown on a substrate is covalently bonded to the underlying substrate material. However, stringent requirements need to be fulfilled in order to achieve a reasonable quality of such multilayer heterostructures, thereby imposing restriction to its universal applicability. For example, direct heteroepitaxy of group III-V or group II-VI materials on Si using conventional epitaxial growth methods is generally not possible due to thermal expansion, polarity and lattice mismatch between the group III-V or group II-VI materials with Si. 
     One way to overcome this is by using van der Waals epitaxy (vdWE) technique. The van der Waals epitaxy (vdWE) technique is based on non-covalent interactions between adatoms and a substrate surface. The non-covalent interaction relaxes the lattice alignment requirement and allows materials with comparatively large mismatch to grow on each other. With its compatibility to the growth of two-dimensional (2D) materials, the vdWE technique has become a material growth method of interest in recent years for advancing the fabrication of semiconducting devices. Unfortunately, a crystalline 2D material typically lacks dangling bonds on its surface and therefore offers very low surface energy and adsorption energy for adatoms during subsequent epitaxial growth. This makes growth of heterostructures using the vdWE technique challenging for achieving uniform, strain-free films, which often results in island-type growths, low growth rates and defective films for the subsequent epitaxial growth. This inadvertently affects device performances of the resultant devices. The integration of a three-dimensional (3D) material on a 2D material is even more challenging given the weak vdW interactions employed in this vdWE technique. The weak vdW interaction leads to very low wetting of a surface of the 2D material by a typical 3D material, resulting in the formation of non-uniform, strained and clustered 3D material films instead of uniform, planar ones. 
     It is therefore desirable to provide a seed layer, a heterostructure comprising the seed layer, and a method of forming a layer of material using the seed layer which address the aforementioned problems and/or provide a useful alternative. Further, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure. 
     SUMMARY 
     Aspects of the present application relate to a seed layer, a method of forming the seed layer, a heterostructure comprising the seed layer, a device comprising the heterostructure, a method of forming a layer of material using the seed layer and a method of enhancing van der Waals (vdW) interaction between adatoms and a surface of the seed layer. 
     In accordance with a first aspect, there is provided a seed layer for inducing nucleation to form a layer of material. The seed layer comprising a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure adapted to create localised electronic states to form electric potential wells for bonding adatoms to a surface of the seed layer via van der Waals (vdW) interaction to form the layer of material, wherein each of the electric potential wells has a potential energy larger in magnitude than surrounding thermal energy to capture adatoms on the surface of the seed layer. 
     By using a seed layer comprising a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure, localised electronic states are created by the disordered atomic structure to form the electric potential wells which act as high energy sites for adsorbing adatoms during a growth of the layer of material via vdW interaction. This results in a stronger interaction between the adatoms and the surface of the seed layer and a higher nucleation density of adatoms on the surface of the seed layer (e.g. when compared with conventional vdW epitaxy), which work together to enhance a wettability of the adatoms on the surface of the seed layer for achieving uniform planar material layer growth. Moreover, the disordered atomic structure of the seed layer can also be tuned from a completely amorphous phase to a nanocrystalline phase for modulating an interaction between an underlying substrate and the layer of material, thereby providing a useful handle for remotely controlling the growth of this layer of material. Further, since growth of the layer of material is derived from the vdW interaction between the adatoms and the seed layer, the seed layer functions as a universal seed layer for allowing any material layer to be grown on any substrate. Still further, a stronger vdW interaction between the surface of the seed layer and the layer of material allows the grown layer of material to be detached from the underlying substrate to create freestanding films which may be advantageous in a design of a heterostructure electronic device. 
     The layer of 2D monolayer amorphous material may comprise a 2D monolayer amorphous carbon. 
     The seed layer may have an optical transparency of more than 98% at a light wavelength between 550 nm to 800 nm. 
     The seed layer may be thermally stable from room temperature up to 700° C., from room temperature up to 600° C., from room temperature up to 500° C., from room temperature up to 400° C., from room temperature up to 300° C., from room temperature up to 200° C., from room temperature up to 100° C. or at a temperature between 600° C. and 700° C., between 500° C. and 700° C., between 400° C. and 700° C., between 300° C. and 700° C., between 200° C. and 700° C., between 100° C. and 700° C., between 20° C. and 700° C., or at 700° C. 
     The seed layer may comprise one or more additional layers of 2D monolayer amorphous material deposited on the layer of 2D monolayer amorphous material to form a multilayer structure of the seed layer. 
     In accordance with a second aspect, there is provided a method of forming a seed layer, the seed layer comprising a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure adapted to create localised electronic states to form electric potential wells for bonding adatoms to a surface of the seed layer via van der Waals (vdW) interaction to form a layer of material, wherein each of the electric potential wells has a potential energy larger in magnitude than surrounding thermal energy to capture adatoms on the surface of the seed layer, the method comprising: growing the seed layer on a substrate using laser-assisted chemical vapour deposition (LCVD). 
     The LCVD enables non-catalytic growth of the seed layer directly on a variety of substrates (metals, semiconductor, insulators, glass and polymers) at a low temperature, by making use of a photolytic decomposition process. The photolytic decomposition refers to the use of one or more photons to induce a chemical reaction of a molecule to break the molecule down into simpler particles. This provides a number of advantages. First, the use of laser assisted CVD allows for direct growth of the seed layer on a substrate of interest and so it bypasses the commonly practiced time-consuming transfer method which is required when the seed layer or the material of interest can be grown only on a specific base substrate. Second, by being able to grow the seed layer on a substrate of interest and bypassing the transfer method, it provides a cleaner surface of the seed layer for subsequent growth of the layer of material since the growth of the layer of material can be formed in-situ in a same CVD or growth process. This reduces potential impurities on the surface of the seed layer, thereby allowing formation of a defect-free uniform planar layer of material on the seed layer. For clarity, it should be appreciated that the subsequent growth of the layer of material is not limited to LCVD. Other suitable growth processes for forming the layer of material (e.g. 2D, 3D or oxide materials) can be used where the growth processes can be implemented in-situ. Third, the LCVD process enables lower temperature growth of the seed layer on the base substrate, thereby retaining a pristine surface and crystallinity of the material of the base substrate for the subsequent growth, particularly if the material of the base substrate has a low thermal stability (e.g. thermally stable at a temperature below 300° C. or 400° C.). Fourth, the low temperature laser assisted CVD process is also compatible with conventional semiconducting processing technology. 
     In accordance with a third aspect, there is provided a heterostructure comprising: a substrate; and a seed layer formed on the substrate, the seed layer comprising a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure adapted to create localised electronic states to form electric potential wells for bonding adatoms to a surface of the seed layer via van der Waals (vdW) interaction, wherein each of the electric potential wells has a potential energy larger in magnitude than surrounding thermal energy to capture adatoms on the surface of the seed layer. 
     The substrate may comprise one of: a metal, a semiconductor, an insulator, glass, a polymer, silicon, silicon carbide, sapphire, a group III-V substrate, a group II-VI substrate or an oxide. 
     Where the substrate is a crystalline substrate, the seed layer may be adapted to screen effects provided by a crystallinity of the crystalline substrate. 
     The heterostructure may comprise a layer of material formed on the seed layer, the layer of material being formed by bonding adatoms of the material to the surface of the seed layer via the van der Waals (vdW) interaction. 
     The layer of material may comprise one or more layers of a 2D material, the 2D material comprises one of: graphene, borophene, boron nitride, a perovskite, a transition metal dichalcogenide or a black phosphorene. 
     The layer of material may comprise one or more layers of a group III-V semiconducting material. 
     The group III-V semiconducting material may comprise one of: GaAs, GaN, AIN, InP and InN. 
     The layer of material may comprise one or more layers of a group II-VI semiconducting material. 
     The group II-VI semiconducting material may comprise one of: CdTe, CdS and ZnS. 
     The layer of material may comprise one or more layers of an oxide. 
     The oxide may comprise one of: hafnium oxide, aluminium oxide, manganese oxide, perovskite or spinel. 
     The seed layer may comprise a 2D monolayer amorphous carbon. 
     In accordance with a fourth aspect, there is provided a device comprising any preceding heterostructure. 
     In accordance with a fifth aspect, there is provided a method of forming a layer of material on a substrate. The method comprising: forming a seed layer on the substrate, the seed layer comprising a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure adapted to create localised electronic states to form electric potential wells for bonding adatoms to a surface of the seed layer via van der Waals (vdW) interaction, wherein each of the electric potential wells has a potential energy larger in magnitude than surrounding thermal energy to capture adatoms on the surface of the seed layer; and forming the layer of material on the seed layer by bonding adatoms of the material to the surface of the seed layer via the van der Waals (vdVV) interaction. 
     The method may comprise varying the disordered atomic structure of the layer of 2D monolayer amorphous material to modulate a strength of the vdW interaction between the adatoms of the material and the surface of the seed layer. 
     The method may comprise: forming a handling layer on the layer of material; and detaching the seed layer from the substrate to form a free-standing film comprising the seed layer and the layer of material. 
     Forming the seed layer on the substrate may comprise growing the seed layer on the substrate using laser-assisted chemical vapour deposition (LCVD). 
     The laser-assisted CVD may be performed at a temperature between room temperature (e.g. 20° C.) and 400° C., between 20° C. and 50° C., between 20° C. and 100° C., between 20° C. and 150° C., between 20° C. and 200° C., between 20° C. and 300° C., between 100° C. and 200° C., between 100° C. and 300° C., between 200° C. and 300° C., between 200° C. and 400° C. or between 300° C. and 400° C. In some embodiments, a temperature range of between 20° C. and 300° C. or between 20° C. and 400° C. may be advantageous as this is complementary to industrial processes given that complementary metal oxide semiconductor (CMOS) technology typically has temperature limitations within these temperature ranges. For example, nanometre size domains in thin film materials may be damaged if they are exposed to high temperatures of above 300° C. or 400° C. In some embodiments, growing the seed layer using LCVD at room temperature may be advantageous since no heating is required for the LCVD growth process and therefore the growth equipment used may not need to be equipped with a heater or suitable heat isolation/containment. Further, eliminating heating for the LCVD growth of the seed layer also reduces the energy consumption and therefore the overall costs of the material growth. 
     In accordance with a sixth aspect, there is provided a method for enhancing van der Waals (vdW) interaction between adatoms and a surface of a seed layer for forming a layer of material on the seed layer. The seed layer comprises a layer of two-dimensional (2D) monolayer, and the method comprises: creating a disordered atomic structure in the seed layer, the disordered atomic structure of the seed layer being adapted to create localised electronic states to form electric potential wells for bonding the adatoms to the surface of the seed layer via the vdW interaction to form the layer of material, wherein each of the electric potential wells has a potential energy larger in magnitude than surrounding thermal energy to capture adatoms on the surface of the seed layer. 
     It should be appreciated that features relating to one aspect may be applicable to the other aspects. Embodiments therefore provide a seed layer comprising a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure, where localised electronic states are created by the disordered atomic structure to form electric potential wells which act as high energy sites for adsorbing adatoms during a growth of the layer of material via vdW interaction. This results in a stronger interaction between the adatoms and the surface of the seed layer and a higher nucleation density of adatoms on the surface of the seed layer, which work together to enhance a wettability of the adatoms on the surface of the seed layer for achieving uniform planar material layer growth. Moreover, the disordered atomic structure of the seed layer can also be tuned from a completely amorphous phase to a nanocrystalline phase for modulating an interaction between an underlying substrate and the layer of material, thereby providing a useful handle for remotely controlling the growth of this layer of material. Further, a stronger vdW interaction between the surface of the seed layer and the layer of material allows the grown layer of material to be detached from the underlying substrate to create freestanding films, which may be advantageous in a design of heterostructure electronic devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the following drawings, in which: 
         FIG.  1    shows a schematic structure of a heterostructure comprising a seed layer in accordance with an embodiment; 
         FIG.  2    shows a schematic of a planar view of a seed layer comprising a monolayer amorphous carbon (MAC) in accordance with an embodiment; 
         FIG.  3    shows a flowchart showing steps of a method for forming the heterostructure of  FIG.  1   ; 
         FIG.  4    shows a flowchart showing steps of a method for forming a free-standing film comprising the seed layer and the layer of material using the heterostructure of  FIG.  1   ; 
         FIG.  5    shows schematic diagrams illustrating the steps of the method for forming the free-standing film in relation to  FIG.  4   ; 
         FIG.  6    shows photographs taken of as-grown MAC films on three distinctive substrates, namely, titanium, glass and copper, in accordance with an embodiment; 
         FIG.  7    shows Raman spectra of the as-grown MAC films of  FIG.  6   ; 
         FIG.  8    shows a diagram illustrating a theoretical simulation of an out-of-plane structural relaxation within a MAC film which induces localised strain within a lattice structure of the MAC film in accordance with an embodiment; 
         FIG.  9    shows a model used for the theoretical simulation of  FIG.  8    overlaid with the modulus squared of the wave functions to show localised electronic distribution in the atomic structure of the MAC; 
         FIG.  10    shows a plot of optical transmission spectrum of the MAC in accordance with an embodiment; 
         FIGS.  11 A and  11 B  show scanning transmission electron microscopy (STEM) images of a seed layer with two different structural variations in accordance with an embodiment, where  FIG.  11 A  shows a STEM image of a MAC film and  FIG.  11 B  shows a STEM image of a nanocrystalline graphene film; 
         FIGS.  12 A and  12 B  show Raman spectra of a MAC before and after a temperature treatment at about 700° C. in accordance with an embodiment, where  FIG.  12 A  shows a Raman spectrum of the MAC before the temperature treatment and  FIG.  12 B  shows a Raman spectrum of the MAC after the temperature treatment; 
         FIG.  13    shows a transmission electron microscopy (TEM) image of the MAC of  FIG.  12 B  after the temperature treatment; 
         FIG.  14    shows a schematic of a heterostructure comprising a two-dimensional (2D) material grown on the seed layer on a substrate in accordance with an embodiment; 
         FIG.  15    shows a schematic of a heterostructure comprising a three-dimensional (3D) material grown on the seed layer on a substrate in accordance with an embodiment; 
         FIGS.  16 A,  16 B and  16 C  show optical images of MoS 2  grown on three different surfaces in accordance with an embodiment, where  FIG.  16 A  shows an optical image of MoS 2  grown on silicon dioxide (SiO 2 ),  FIG.  16 B  shows an optical image of MoS 2  grown on a monolayer of MAC on SiO 2 , and  FIG.  16 C  shows an optical image of MoS 2  grown on a few layers of MAC on SiO 2 ; 
         FIGS.  17 A and  17 B  show scanning electron microscopy (SEM) images of MoS 2  grown using a sapphire substrate in accordance with an embodiment, where  FIG.  17 A  shows SEM images of MoS 2  grown directly on the sapphire substrate and on a monolayer of MAC on the sapphire substrate, and  FIG.  17 B  shows a zoom-in SEM image of the MoS 2  grown on the monolayer of MAC on the sapphire substrate; 
         FIGS.  18 A and  18 B  show scanning electron microscopy (SEM) images of MoS 2  grown using a sapphire substrate in accordance with an embodiment, where  FIG.  18 A  shows SEM images of MoS 2  grown directly on the sapphire substrate and on a few layers of MAC on the sapphire substrate, and  FIG.  18 B  shows a zoom-in SEM image of the MoS 2  grown on the few layers of MAC on the sapphire substrate; and 
         FIG.  19    shows an atomic force microscopy (AFM) image of In 2 Se 3  grown on a monolayer of MAC on a SiO 2  substrate in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments relate to a seed layer, a method of forming the seed layer, a heterostructure comprising the seed layer, a device comprising the heterostructure, a method of forming a layer of material using the seed layer and a method of enhancing vdW interaction between adatoms and a surface of the seed layer. 
     It is appreciated that in the present application, the use of the singular includes the plural unless specifically stated otherwise. It should 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. Further, the use of the term “including”, “comprising”, and “having” as well as other forms, such as “include”, “comprise”, “have” are not considered limiting. 
     In the present application, the device and/or heterostructure as described herein may be operable in various orientations, and thus it should be understood that the terms “top”, “base”, “underlying” etc. when used in the following description are used for convenience and to aid understanding of relative positions or directions, and not intended to limit the orientation of the device and/or heterostructure. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     In the present embodiments, by using a seed layer comprising a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure where localised electronic states are created by the disordered atomic structure to form electric potential wells, the seed layer is provided with high energy sites for adsorbing adatoms during a growth of the layer of material via vdW interaction. The localised electronic states refer to a distribution of electronic states within the 2D monolayer amorphous material which are not extended to overlap with one another. Particularly, in a disordered material system, these localised electronic states are sufficiently isolated from one another which may lead to an absence of electrical conduction of the 2D monolayer amorphous material. An electric potential well formed by the localised electronic states of the 2D monolayer amorphous material refers to a trapping site which has a potential energy larger in magnitude than available surrounding thermal energy so as to capture adatoms on the surface of the seed layer, preferably at locations of these electric potential wells. These electric potential wells formed by the localised electronic states created by the disordered atomic structure of the 2D monolayer amorphous material provide a strong interaction between the adatoms and the surface of the seed layer and a high nucleation density of adatoms on the surface of the seed layer, which work together to enhance a wettability of the adatoms on the surface of the seed layer for achieving a subsequent uniform planar material layer growth. In the present case, enhancing the wettability of the adatoms refers to improving an attraction force between the adatoms and the surface of the seed layer so that this attraction force is stronger than an attractive interaction force between the adatoms. Enhancing a wettability of the adatoms leads to formation of a uniform distribution of adatoms on the surface of the seed layer, instead of formation of clusters of adatoms on the surface. Moreover, the disordered atomic structure of the seed layer can also be tuned from a completely amorphous phase to a nanocrystalline phase for modulating an interaction between an underlying substrate and the layer of material, thereby providing a useful handle for remotely controlling the growth of this layer of material. Further, a strong vdW interaction between the surface of the seed layer and the layer of material allows the grown layer of material to be detached from the underlying substrate to create freestanding films which may be advantageous in a design of a heterostructure electronic device. In the current context, the term “amorphous material” refers to a material that lacks the long-range order which is typical of a crystalline material. The term “monolayer” refers to a one-atom thick layer, which may range from a few angstroms (Å) to a few nanometres thick. 
       FIG.  1    shows a schematic structure of a heterostructure  100  in accordance with an embodiment. The heterostructure  100  comprises a seed layer  102  formed on a substrate  104 . As shown in  FIG.  1   , the seed layer  102  is formed directly on the substrate  104  (i.e. the seed layer  102  is formed on top of and adjacent to the substrate  104 ). The substrate  104  provides structural support for the seed layer  102 . The heterostructure  100  also includes a layer of material  106  formed on the seed layer  102 . The layer of material  106  includes any material of interest and this will be further described in relation to  FIGS.  14  and  15    below. The seed layer  102  comprises a layer of two-dimensional (2D) monolayer amorphous material having a disordered atomic structure. The disordered atomic structure is adapted to create localised electronic states to form electric potential wells for bonding adatoms to a surface of the seed layer  102  via van der Waals (vdW) interaction to form the layer of material  106 . The seed layer  102  comprising the layer of 2D monolayer amorphous material may be formed by any 2D material, as long as the seed layer  102  has a disordered atomic structure which creates localised electronic states for forming electric potential wells for enhancing the vdW interaction between the adatoms and the surface of the seed layer  102 . 
     In the embodiments as described below, a monolayer amorphous carbon (MAC) is used as an exemplary 2D monolayer amorphous material for the seed layer  102 .  FIGS.  2  to  19    will be described in relation to using one or more layers of the MAC as the seed layer  102 . A MAC comprises sp 2 -bonded carbon lattice which retains similar vdW interaction as a graphene monolayer, but provides additional vdW interaction from its disordered atomic structure which creates localised electronic states that can act as a driving force for aligning adatoms in-plane to a surface of the MAC. When using the MAC as the seed layer  102 , this results in a higher surface wettability (e.g. as compared to graphene or other crystalline 2D materials) of the adatoms on the surface of the seed layer  102 , thereby enabling uniform and planar growth of the layer of material  106 . This makes MAC, in contrast to e.g. graphene, an ideal candidate as the seed layer  102  for epitaxial and/or non-epitaxial growth of a variety of material films. 
       FIG.  2    shows a schematic of a planar view  200  of a seed layer comprising the MAC. 
     As shown in  FIG.  2   , the MAC comprises a disordered atomic arrangement with a continuous network of disordered sp 2  carbon (C) atoms in two dimensions (2D) without any grain boundaries (homogeneous). This is in contrast to conventional polycrystalline graphene which includes ordered crystalline domains separated with grain-boundaries (inhomogeneous). Due to the lack of grain boundaries in a MAC seed layer  102 , the disclosed MAC seed layer  102  is ultra-strong, making it suitable for applications that may require deformation such as bending and stretching. In the MAC, a ratio of hexagonal carbon rings to a total number of carbon rings (i.e. the total number of hexagonal and non-hexagonal carbon rings) may be less than 1. 
       FIG.  3    shows a flowchart showing steps of a method  300  for forming the heterostructure  100  of  FIG.  1   . 
     In a step  302 , the seed layer  102  is formed on the substrate  104 . In the present embodiment, the seed layer  102  comprises a monolayer amorphous carbon (MAC), and the substrate  104  comprises a sapphire substrate. In the present embodiment, the MAC is formed using a laser-assisted chemical vapour deposition (LCVD) process with hydrocarbons as precursors (e.g. CH 4 , C 2 H 2  etc.) at room temperature. Hydrogen gas (H 2 ) and Argon gas (Ar) may also be mixed with the precursor. In this LCVD process, the laser functions both as an energy source to breakdown the precursor gas in a process called photolytic decomposition, and as a local heat source. In the present embodiment, the LCVD process for producing the MAC seed layer  102  uses the following parameters: (i) process gas: C 2 H 2 ; (ii) chamber pressure: 2×10 −2  mbar; (iii) laser fluence: 70 mJ/cm; (iv) growth time: 1 min; (v) plasma power: 5 W. Although the LCVD process is used for forming the MAC in the present embodiment, it will be appreciated that the LCVD process can also be used for forming non-carbon based seed layer  102  in other embodiments. 
     The above exemplary process employs the use of acetylene (C 2 H 2 ) within the growth chamber for the growth process. The gas pressure within the chamber during the growth is controlled at 2×10 −2  mbar throughout. This gas is in the presence of a plasma generator operating at 5 W power. The growth starts when a 248 nm excimer laser is exposed on the surface of the sapphire substrate  104  with a fluence of 70 mJ/cm with a pulse frequency of 50 Hz. The laser exposure time (i.e., growth duration) is set at 1 min to obtain a continuous MAC seed layer  102  on the substrate  104 . In this growth, the stage heater is not used. Multiple parameters disclosed herein may be adjusted, for controlling and/or changing the properties of the disclosed MAC seed layer  102  including, but not limited to, hydrocarbons as precursors, precursor mixes, adjustments to the photolytic decomposition process and equipment, temperature regulations, substrate temperature adjustment, the change in C value, change in number of atomic layers, change in sp 2  to spa ratio, and change in adhesion to the substrate  104 . In the present embodiment, a thickness of the MAC seed layer  102  is designed to be at approximately one atomic layer thick. 
     Further, it should be appreciated that use of the photolytic decomposition approach for forming the seed layer  102  as described above is distinct from the typical approach of forming a 2D material film using e.g. thermal CVD (TCVD). Particularly, TCVD requires a hot substrate for the chemical reactions of bond breaking and bond forming of adatoms to occur on a surface of the substrate. However, a temperature required for such reactions to occur is typically much higher than the crystallisation temperature of a 2D material. This means that at the minimum growth temperature required to form a 2D material film on the surface of the substrate, the atoms of the 2D material deposited on the substrate surface are highly mobile (e.g. surface diffusion) and will reorder themselves leading to some degree of crystallisation during formation of the 2D material. As a result, the 2D material film formed will be in varying degree of crystallinity, and cannot be completely or fully amorphous. In contrast, by using the photolytic decomposition approach, energy from the laser breaks the bonds of the precursor gas and provides additional energy for subsequent 2D material film formation when the atoms of the 2D material are deposited on the substrate surface at a lower temperature (e.g. at room temperature, or at a temperature below the crystallisation temperature). 
     Therefore, by using the photolytic decomposition approach at low temperatures, the deposited atoms of the 2D material are less mobile and are unlikely to move after landing on the surface of the substrate. This limitation in atomic motions of the atoms (surface diffusion) prevents crystal formation of the 2D material. A monolayer amorphous film can therefore be formed. 
     By using the photolytic decomposition method for forming the MAC seed layer  102  as described above, a number of advantages can be provided. First, the MAC seed layer  102  as synthesised by LCVD can be integrated with existing semiconductor processing technology. Particularly, LCVD is an industrially scalable process which can achieve high throughput of large area films. Therefore the LCVD process for seed layer formation can be integrated easily with current semiconductor processing technology, making the process industrially compatible and scalable. In addition, LCVD is an ultrafast deposition technology, where an entire surface of the substrate  104  can be covered with a MAC film in under 60 seconds. LCVD is thus more efficient than the widely employed atomic layer deposition (ALD) process. 
     Second, using LCVD means that the MAC seed layer  102  can be synthesized at low temperatures of less than 300° C. (for example as low as 200° C. or even at room temperature), which is compatible with silicon-based technologies. Also, in contrast to the growth of graphene, the cost of MAC growth using LCVD is significantly lower since less energy is required for the LCVD growth as compared to the conventional thermal chemical vapor deposition of graphene which requires a temperature of about 1000° C. Further, lowering of synthesis temperature (e.g. at a temperature between 20° C. to 150° C.) may make it possible to enable direct MAC growth on polymeric substrates used for OLED and flexible electronics. The low temperature growth of MAC as the seed layer  102  is also advantageous as it minimises a disruption of a lattice or surface reconstruction of a single-crystal substrate, retaining a pristine and smooth interface between the seed layer  102  and the substrate  104 . 
     Third, with the low-temperature photolytic growth of MAC by LCVD, direct growth of a MAC seed layer  102  can be performed on a variety of substrates (including Si, single crystals, polycrystalline, metals, glass, polymers and others). Also, subsequent growth of the layer of material  106  on the seed layer  102  is governed by the vdW interaction of a surface of the MAC seed layer and adsorbing adatoms, thereby minimising a role of the substrate  104  on this subsequent material growth. Particularly, one or more layers of MAC can be adapted to screen the underlying crystalline information of the substrate  104  and hence dominating the subsequent growth mechanism. 
     In a step  304 , the layer of material  106  is formed or deposited on the seed layer  102 . Given the stronger vdW interaction between the adatoms and the surface of the seed layer  102  as provided by the disordered atomic structure of the seed layer  102 , a variety of materials can be used for forming the layer of material  106 . This is further described in relation to  FIGS.  14  and  15   . It will therefore be appreciated that a large number of growth or deposition techniques can be used for forming the layer of material  106 , depending on the material used for the layer of material  106 . Examples of deposition techniques which may be applied include molecular beam epitaxy (MBE), atmospheric pressure CVD (APCVD), metal organic CVD (MOCVD), plasma enhanced CVD (PECVD), thermal CVD (TCVD) and atomic layer deposition (ALD). Embodiments of different materials used for forming the layer of material  106  are discussed in relation to  FIGS.  16 A to  19   . 
       FIG.  4    shows a flowchart showing steps of a method  400  for forming a free-standing film comprising the seed layer  102  and the layer of material  106  using the heterostructure  100  of  FIG.  1   . 
     In a step  402 , a handling layer is formed on the layer of material  106 . In other words, the handling layer is formed on and adjacent to the layer of material  106 . The handling layer includes a metal stressor layer, a flexible tape layer or a layer of adhesive material which can have a stronger adhesion to the underlying layer of material  106  as compared to the adhesion between the seed layer  102  and the substrate  104 . 
     In a step  404 , the seed layer  102  is detached from the substrate  104  to form a free-standing film. This is achieved by exfoliating or peeling the layer of material  106  and the seed layer  102  from the substrate  104  to form the free-standing film, and subsequently removing the handling layer which was formed or attached to the layer of material  106 . 
     The exfoliation is governed by the stronger interaction between the seed layer  102  with the layer of material  106  as compared to that with the substrate  104 . The stronger interface between the seed layer  102  and the layer of material  106 , and the non-covalent bonding of the seed layer  102  with the underlying substrate  104  helps to exfoliate the free-standing film from the substrate  104 . This is advantageous as the free standing film comprising the seed layer  102  and the layer of material  106  can be isolated for use in for example flexible and transparent optoelectronic devices, while the substrate  104  can be reused. 
       FIG.  5    shows schematic diagrams  500  illustrating the steps of the method  400  for forming the free-standing film of  FIG.  4   . In the present embodiment, a two-layer MAC is formed on a substrate  510  and is used as the seed layer  512 . A two-layer 2D material is formed on the seed layer  512  as the layer of material  514 . 
     As shown by the schematic diagram  502 , a handling layer  516  is formed on top of the layer of material  514 . This corresponds to the step  402  as described above. 
     As shown by the schematic diagram  504 , exfoliation or peeling of the layer of material  514  and the seed layer  512  from the substrate  510  is performed using the handling layer  516 . The detached free-standing layer  518  comprises the layer of material  514  and the seed layer  512 . 
     As shown by the schematic diagram  506 , the handling layer  516  is subsequently removed from the free-standing layer  518 . The handling layer  516  may be removed, for example, by the following ways. Where the handling layer  516  is a metal stressor layer, it may be removed by dipping the handling layer  516  in a metal etchant. Where the handling layer  516  is a flexible tape layer, the flexible tape layer may include a thermal release or a UV release adhesion that can be removed by heating or exposing the tape to the UV light, respectively. 
       FIG.  6    shows photographs  600  taken of as-grown MAC films on three distinctive substrates, namely, titanium, glass and copper. As shown in  FIG.  6   , the photograph  602  shows an as-grown MAC film on a titanium substrate, the photograph  604  shows an as-grown MAC film on a glass substrate, and the photograph  606  shows an as-grown MAC film on a copper substrate. At least from the photograph  604 , it is clear that the MAC film as deposited is transparent in visible light. 
       FIG.  7    shows Raman spectra of the as-grown MAC films of  FIG.  6   . The Raman spectrum  702  is obtained using the MAC film grown on the glass substrate, the Raman spectrum  704  is obtained using the MAC film grown on the titanium substrate, and the Raman spectrum  706  is obtained using the MAC film grown on the copper substrate. As shown in all of the Raman spectra  702 ,  704 ,  706  of  FIG.  7   , there is an absence of a 2D peak (at about 2700 cm −1 ) which is typical of a crystalline graphene monolayer. In contrast, all of the Raman spectra  702 ,  704 ,  706  show broad G peaks  708  (at about 1600 cm −1 ) and D peaks  710  (at about 1350 cm −1 ). The broadening of D and G peaks usually indicates a transition from nanocrystalline graphene to amorphous film. The Raman spectra  702 ,  704 ,  706  also reveal a DIG-ratio in a range of about 0.5 to 1. This D/G-ratio combined with the absence of the 2D peak distinguishes the disordered atomic structure of the MAC from the structures of 2D graphene and diamond. The Raman spectra  702 ,  704 ,  706  also verified the growth of the as-grown MAC films on these three distinctive substrates. 
       FIG.  8    shows a diagram  800  illustrating a theoretical simulation of an out-of-plane structural relaxation within a MAC film  802  which induces localised strain in a lattice structure of the MAC film  802 . In the present theoretical simulation, atomic coordinates of the atoms of the MAC film  802  in the model are initially arranged in a flat 2D plane. By taking into consideration the interaction forces between these atoms, atomic rearrangement of these atoms occurs in a 3D space, and the atomic coordinates of the atoms of the MAC film  802  take up their new equilibrium positions where the structure of the MAC film  802  is in its most stable configuration with the lowest internal energy. As shown in the diagram  800 , the MAC film  802  simulated is of a monolayer and has a thickness  804  of about 6.5 Å. The amorphous or disordered atomic structure of the MAC film  802  generates a strained 2D lattice with localised electronic distribution, thereby providing a surface with relatively higher energy. Such surface initiates a strong interaction with the adsorbing adatoms resulting in higher surface wettability required for uniform and planar 2D and/or 3D material film formation. This helps to overcome a low wetting of 3D material films on a 2D surface for vdW epitaxy due to the intrinsic lower surface energy of a surface of a 2D crystalline material. 
     Further, the higher surface energy of the MAC  802  due to its disordered atomic arrangement and enhanced vdW interaction results in a high number of nucleation sites for adsorbed adatoms on the MAC seed layer. A high number of nucleation sites (or higher nucleation density) can substantially lower the growth rate and temperature requirement for the subsequent material layer growth, thereby making the growth process more energy and cost efficient. Moreover, the enhanced vdW interaction between the MAC  802  (i.e. the seed layer) and the layer of material (or epilayer) results in a stronger interface which is stable even during a subsequent high temperature growth process. This ensures uniformity of subsequent formation of the planar layer of material and prevents or reduces formation of islands and/or clusters during this subsequent growth of the layer of material. It is noted that the formation of islands and/or clusters in the layer of material results in non-planar active layer material films which is detrimental to a device performance of a subsequent device formed using such a non-planar material layer. 
       FIG.  9    shows a model  900  used for the theoretical simulation of  FIG.  8   . The model  900  is overlaid with the modulus squared of the wave functions to show localised electronic distribution  902  in the atomic structure of the MAC  802 . 
     As shown in the model  900  of  FIG.  9   , the MAC film is a single-atom thick carbon film having a mixture of hexagonal and non-hexagonal rings in its structure. The rings are fully connected to each other, forming a network of polygons in a large area film whose scale is at least in microns. The ratio of a number of hexagonal rings to a total number of carbon rings (i.e. a total number of hexagonal and non-hexagonal rings) is a measure of crystallinity (or amorphousity), C. Non-hexagons are in a form of 4-, 5-, 7-, 8-, 9-membered rings. A 5-membered ring  904  and a 7-membered ring  906  are shown in  FIG.  9   , in contrast with the regular 6-membered ring  908  which is typical of crystalline graphene. Disclosed embodiments may support a C value range between and including 0.5 to 0.8. This is different from graphene where C=1 for pure hexagonal network. 
       FIG.  10    shows a plot  1000  of optical transmission spectrum of the MAC  1002  in accordance with an embodiment. 
     The plot  1000  illustrates an optical transparency of the MAC  1002  over a range of light wavelengths. As shown in  FIG.  10   , the optical transparency is at ˜98.1% at a light wavelength of 550 nm, increasing in transparency with increasing light wavelength. Thus, embodiments of the present invention provide a MAC having an optical transparency equal to or greater than 98% at a wavelength of 550 nm or higher. The disclosed MAC differs from graphene. The line  1004  shows a theoretical limit of 97.7% for an optical transparency of graphene. It is therefore evidenced from at least the plot  1000  that the MAC  1002  of present embodiments exhibits higher optical transparency than graphene at a wavelength of about 550 nm or higher. Notably the transparency of the MAC  1002  does not decrease rapidly at short wavelengths (&lt;400 nm). This may partly be due to less contamination in the growth process of the MAC  1002  since the MAC  1002  can be grown on any substrates without using a transfer method. The high optical transparency of the MAC within the visible light range (˜98.1% at 550 nm with increasing optical transparency at higher wavelengths) makes MAC an ideal candidate for a seed layer on transparent substrates (e.g. glass or suitable polymers) for forming subsequent active semiconducting films for transparent devices. 
       FIGS.  11 A and  11 B  show scanning transmission electron microscopy (STEM) images  1100 ,  1110  of a seed layer with two different structural variations in accordance with an embodiment, where  FIG.  11 A  shows a STEM image  1100  of a MAC film and  FIG.  11 B  shows a STEM image  1110  of a nanocrystalline graphene film. The “white” clusters  1112  as shown in  FIG.  11 B  are related to contamination absorbed on the surface of the nanocrystalline graphene film and is not related to crystallinity of its atomic structure. 
     A wide range of atomic structural variation within a carbon-based seed layer is possible, from a completely amorphous layer (e.g. a MAC) to a nanocrystalline sp 2 -carbon layer (e.g. a nanocrystalline graphene layer) depending on the synthesis conditions. Moreover, the seed layer can also be formed ranging from a monolayer to a multilayer stack on a substrate. Such structural variations can tune the vdW interaction between the seed layer and the layer of material (or the epilayer), and can remotely modulate an interaction between the substrate and the adatoms for forming the layer of material during growth. For example, by tuning the crystallinity of the seed layer from completely amorphous to nanocrystalline, more interaction between the substrate and the adatoms can be achieved because the screening of a crystallinity effect of the substrate is provided by the electric potential wells of the disordered atomic structure of the 2D amorphous seed layer. 
     An example of tuning the crystallinity of the carbon based seed layer can be done by using similar laser-based growth conditions as described in relation to the step  302 , but with the use of e.g. methane precursor gas and a copper foil substrate. For example, for forming the nanocrystalline carbon film as shown in  FIG.  11 B , the temperature of the copper foil can be set in a range of 500° C. to 600° C., while for forming a fully amorphous film like that shown in  FIG.  11 A , the temperature of the copper foil can be set at less than 400° C. This is because a higher substrate temperature during growth leads to a more crystalline material. 
       FIGS.  12 A and  12 B  show Raman spectra  1200 ,  1210  of a MAC before and after a temperature treatment at about 700° C. in accordance with an embodiment, where  FIG.  12 A  shows the Raman spectrum  1200  of the MAC before the temperature treatment and  FIG.  12 B  shows the Raman spectrum  1210  of the MAC after the temperature treatment. 
       FIG.  12 A  shows raw data  1202  obtained from Raman spectroscopy of the MAC before the temperature treatment, fitted with a D band  1204  and a G band  1206 , while  FIG.  12 B  shows raw data  1212  obtained from Raman spectroscopy of the MAC after the temperature treatment, fitted with a D band  1214  and a G band  1216 . As shown in the Raman spectra  1200 ,  1210 , the shapes of the D bands  1204 ,  1214  and the G bands  1206 ,  1216  and their D/G ratios are similar. This verifies that there is no observable change in the crystallinity or the grain size of the MAC after undergoing the heat treatment at about 700° C. The MAC is therefore thermally stable at a high temperature of ˜700° C., rendering the MAC a stable seed layer for a subsequent high temperature growth of the layer of material. It will be appreciated that since the MAC is thermally stable at a temperature of ˜700° C., it is also thermally stable at any temperature that is less than 700° C. 
       FIG.  13    shows a transmission electron microscopy (TEM) image  1300  of the MAC of  FIG.  12 B  after the temperature treatment. The TEM image  1300  of the MAC has a size of 10×10 nm 2 , where clusters  1302  as shown in  FIG.  13    are contamination that covers some areas of the MAC. The contamination clusters  1302  may be formed due to the transfer process from a substrate (e.g. a copper foil) to the TEM grid for taking this TEM image  1300 , and are not related to the crystallinity of the MAC. The inset  1304  of  FIG.  13    is the Fourier transform of the TEM image  1300  and shows a diffraction pattern of the TEM image  1300 . Particularly, the inset  1304  shows amorphous halo rings, instead of a sharp ring or an individual spot which indicates nanocrystalline graphene or polycrystalline graphene respectively. 
     In addition to the MAC being a thermally stable layer, the MAC also has a high thermal conductivity which enables it to function as a heat spreading layer in a heterostructure comprising active semiconducting epilayers for transmitting heat to a heat sink. This is advantageous as thermal management is an essential aspect of thin-film devices such as LEDs. The ability of the MAC to spread heat quickly helps to avoid overheating and deterioration of device performances. 
       FIG.  14    shows a schematic of a heterostructure  1400  comprising a layer of two-dimensional (2D) material  1402  grown on a seed layer  1404  on a substrate  1406  in accordance with an embodiment. The heterostructure  1400  has a similar structure as the heterostructure  100  as shown in  FIG.  1    and can be formed or fabricated using the method  300  of  FIG.  3   . 
     The seed layer  1404  comprises a monolayer of amorphous 2D material (e.g. MAC in the present embodiment, but other monolayer 2D amorphous materials can also be used) grown directly on the substrate  1406 . The substrate  1406  in the present embodiment comprises SiO 2  but other substrates such as Si, SiC, sapphire, a group III-V material, a group II-VI material, oxides etc. can also be used. The seed layer  1404  grown on the substrate  1406  functions to stabilize the subsequent growth of the layer of 2D material  1402  using the strong vdW interaction between adatoms of the 2D material layer  1402  and a surface of the seed layer  1404 . This advantageously aids to circumvent the stringent requirements of using specialized substrate for stable growth of the layer of 2D material  1402 . 
     Although  FIG.  14    shows a monolayer amorphous carbon (MAC) as the seed layer  1404 , multilayers of MAC can also be employed by either a direct growth or a transfer method. The layer of 2D material  1402  grown includes one or more layers of monolayer 2D amorphous film, 2D crystalline film, graphene, black phosphorene, borophene, hexagonal boron nitride (hBN) or boron nitride, transition metal dichalcogenide (TMD), perovskite and/or boron phosphide (BP). The layer of 2D material  1402  can be grown on the seed layer  1404  using deposition techniques such as metal-organic chemical vapour deposition (MOCVD), thermal chemical vapour deposition (TCVD), plasma enhanced chemical vapour deposition (PECVD), atomic layer deposition (ALD). The 2D-2D composite structure (i.e. the free-standing structure comprising the layers  1402 ,  1404 ) can be detached from the substrate  1406  after growth to form a freestanding stack which can be further integrated in a fabrication process of thin-film and flexible optoelectronic devices. 
       FIG.  15    shows a structure of a heterostructure  1500  comprising a layer of three-dimensional (3D) material  1502  grown on a seed layer  1504  on a substrate  1506  in accordance with an embodiment. The heterostructure  1500  has a similar structure as the heterostructure  100  as shown in  FIG.  1    and can be formed or fabricated using the method  300  of  FIG.  3   . The difference between the heterostructure  1500  and the heterostructure  1400  is that the layer of material  1502  grown on the seed layer  1504  is a layer of 3D material, in contrast to the layer of 2D material  1402  comprised in the heterostructure  1400 . 
     Different embodiments including different types of 3D materials and substrates used are described below. 
     (i) Integration of CMOS Compatible Substrate (e.g. Si or Ge) with III-V Semiconductors Planar Thin Films 
     In this embodiment, a layer of MAC is formed on a Si or Ge substrate  1506 . The MAC functions as a seed layer  1504  for subsequent epitaxial or non-epitaxial growth of one or more layers  1502  of III-V semiconductors (such as GaAs, GaN, AIN, InP, InN etc.) using existing deposition techniques (such as MOCVD, TCVD, PECVD, ALD etc.). The III-V semiconducting materials can be used as active layers for applications such as light emitting diodes (LEDs), infrared (IR) sensors, photodetectors and other optoelectronic devices. 
     ii) II-VI Semiconductors Thin Films 
     An ability to integrate II-VI thin film semiconductors (e.g. CdTe, CdS, ZnS etc.) on arbitrary substrates (e.g. Si, semiconducting materials, glass, metal foils, polymers etc.) is advantageous for solar cells, photovoltaics and aerospace applications. As discussed above, achieving a uniform planar layer of material by vdW epitaxy is challenging due to the low wettability on epitaxial surface. In present embodiments, a layer of MAC  1504 , which functions as the seed layer  1504 , can be formed on the substrate  1506  for epitaxial or non-epitaxial growth of the layer  1502  of 3D planar films of II-VI semiconductors. Examples of 3D group II-VI semiconducting materials films include CdTe, CdS, ZnS etc. 
     iii) Oxide Thin Films 
     Oxide thin films including simple metal oxides (Hf 2 O 3 , Al 2 O 3 , MnO etc.) and complex oxides (perovskite, spinel etc.) play a significant role in a variety of electronic, spintronic, magnetoelectric and energy storage devices owing to their functional characteristics. These oxides may function as dielectrics, piezoelectrics, pyroelectrics etc. Using a similar scheme as shown in  FIG.  15   , one or more layers of oxides  1502  can be deposited using MAC as the seed layer  1504  on the substrate  1506 . The substrate  1506  in this application can be any arbitrary substrate such as Si, semiconducting materials, glass, metal foils, polymers etc. The deposited layer of oxide film  1502  can also be exfoliated to obtain freestanding membranes using the method  400  as described in relation to  FIGS.  4  and  5   . 
       FIGS.  16 A,  16 B and  16 C  show optical images  1600 ,  1610 ,  1620  of MoS 2  grown on three different surfaces in accordance with an embodiment, where  FIG.  16 A  shows the optical image  1600  of MoS 2  grown on silicon dioxide (SiO 2 ),  FIG.  16 B  shows the optical image  1610  of MoS 2  grown on a single layer of MAC on SiO 2 , and  FIG.  16 C  shows the optical image  1620  of MoS 2  grown on a few layers of MAC on SiO 2 . 
     In the present embodiments, the MAC is first grown on the SiO 2  substrate using the LCVD method as described in relation to the step  302  of  FIG.  3   . The growth conditions of the MAC on SiO 2  substrate are the same as that described for the sapphire substrate in relation to the step  302 , and these are not repeated here for succinctness. The one or more MAC layers acts as a seed layer for the subsequent growth of the MoS 2  layer. The layer of MoS 2  is a 2D material that was grown by thermal CVD over the one or more MAC layers. The parameters for the thermal CVD growth of the MoS 2  layer is as follows: MoO 3 +S, atmospheric pressure chemical vapour deposition (APCVD), 750° C., 20 sccm Ar and a 10-minute growth time. 
     As evidenced in  FIGS.  16 A,  16 B and  16 C , the layer of MoS 2  grows differently on the three different surfaces (i.e. without MAC, a monolayer MAC and a few layers of MAC, respectively). As shown in the optical image  1600  of  FIG.  16 A , standard MoS 2  grown on SiO 2  forms triangular crystals  1602 . The optical image  1610  of  FIG.  16 B  shows that using a single MAC layer as the seed layer results in the growth of MoS 2  crystals  1612  having irregular shapes and multilayer centres. As shown in the optical image  1620  of  FIG.  16 C , using a few layers of MAC as the seed layer results in a high density nucleation of MoS 2  crystals. 
       FIGS.  17 A and  17 B  show scanning electron microscopy (SEM) images  1702 ,  1704 ,  1710  of MoS 2  grown using a sapphire substrate in accordance with an embodiment, where  FIG.  17 A  shows SEM images  1702 ,  1704  of MoS 2  grown directly on the sapphire substrate and on a monolayer of MAC on the sapphire substrate respectively, and  FIG.  17 B  shows a zoom-in SEM image  1710  of the MoS 2  grown on the monolayer of MAC on the sapphire substrate.  FIGS.  18 A and  18 B  show scanning electron microscopy (SEM) images  1802 ,  1804 ,  1810  of MoS 2  grown using a sapphire substrate in accordance with an embodiment, where  FIG.  18 A  shows SEM images  1802 ,  1804  of MoS 2  grown directly on the sapphire substrate and on a few layers of MAC on the sapphire substrate respectively, and  FIG.  18 B  shows a zoom-in SEM image  1810  of the MoS 2  grown on the few layers of MAC on the sapphire substrate. 
     In the present embodiments, the MAC is first grown on the sapphire substrate using the LCVD method as described in relation to the step  302  of  FIG.  3   . The one or more MAC layers acts as a seed layer for the subsequent growth of the MoS 2  layer. The growth conditions of the MAC on the sapphire substrate have been previously described in relation to the step  302 , and these are not repeated here for succinctness. The layer of MoS 2  is a 2D material that was grown by thermal CVD over the one or more MAC layers. The parameters for the thermal CVD growth of the MoS 2  layer is as follows: MoO 3 +S, atmospheric pressure chemical vapour deposition (APCVD), 850° C., 20 sccm Ar, 5 minute of growth time. 
     As evidenced in  FIGS.  17 A,  17 B,  18 A and  18 B , the layer of MoS 2  grows differently on the three different surfaces (i.e. without MAC, a monolayer MAC and a few layers of MAC, respectively). As shown in the SEM images  1702  and  1802  of  FIGS.  17 A and  18 A  respectively, standard MoS 2  grown on sapphire forms triangular crystals  1706 ,  1806 . The SEM image  1710  of  FIG.  17 B  shows that using a single MAC layer as the seed layer results in a high density nucleation of MoS 2  crystals with triangular shape  1712 . As shown in the SEM image  1810  of  FIG.  18 B , using a few layers of MAC as the seed layer results in a high density nucleation of MoS 2  crystals with dendritic shapes  1812 . 
       FIG.  19    shows an atomic force microscopy (AFM) image  1900  of indium selenide (In 2 Se 3 ) grown on a monolayer of MAC on a SiO 2  substrate in accordance with an embodiment. 
     The MAC is first grown or transferred on the SiO 2  substrate, where the MAC functions as a seed layer for subsequent growth of In 2 Se 3 . In 2 Se 3  is a 2D material which is subsequently grown on the MAC using a molecular beam epitaxy (MBE) technique. In the present embodiment, In 2 Se 3  was grown in a MBE chamber with a base pressure˜6×10 −1 ° Torr. Ultrapure In 2 Se 3  powder (99.99%) was evaporated from a crucible heated by an electron beam source with the temperature maintained at 150° C. The chamber pressure during growth was ˜6×10 −9  Torr. 
     The layer of In 2 Se 3  grows differently when using a single layer of MAC as a seed layer as compared to growing the layer of In 2 Se 3  on graphene or directly on the SiO 2  substrate without the MAC. As shown in the AFM image  1900 , the In 2 Se 3  crystals  1902  formed using a single layer of MAC as the seed layer retain the triangular facets with a thickness ranging from a few A (i.e. a thickness of a monolayer) to about 7 nm thick. On the other hand, if In 2 Se 3  is grown on a layer of graphene on SiO 2  substrate, the In 2 Se 3  crystals as grown will be of a monolayer thickness with triangular shapes. If In 2 Se 3  is grown directly on the SiO 2  substrate, the In 2 Se 3  crystals as grown have a highly disordered bulk (3D) structure. 
     Alternative embodiments of the invention include: (i) the seed layer  102  comprises one or more layers of 2D amorphous material selected from one or more of: amorphous MoS 2 , amorphous In 2 Se 3 , amorphous transition metal dichalcogenides, amorphous black phosphorene, amorphous borophene, amorphous boron nitride; (ii) the substrate  104  selected from one of: Si, SiC, sapphire, a group III-V material, a group II-VI material, oxides semiconducting materials, glass, metals and polymers; (iii) the layer of material  106  selected from a 2D material or a 3D material, and examples of a 2D material and a 3D material are provided in relation to  FIGS.  14  and  15   ; and (iv) the layer of material  106  can be formed by various deposition techniques such as LPCVD, APCVD, MOCVD, TCVD, PECVD, MBE and ALD. 
     Although only certain embodiments of the present invention have been described in detail, many variations are possible in accordance with the appended claims. For example, features described in relation to one embodiment may be incorporated into one or more other embodiments and vice versa.