Abstract:
A method for growing a transition metal dichalcogenide on a substrate, the method including providing a growth substrate having a first side and a second side opposite the first side; providing a source substrate having a first side and a second side opposite the first side; depositing a transition metal oxide on at least a portion of the first side of the source substrate; combining the growth substrate with the source substrate such that the first side of the growth substrate contacts the transition metal oxide, the combining producing a substrate stack; exposing the substrate stack to a chalcogenide gas, whereby the transition metal oxide reacts with the chalcogenide gas to produce a layer of a transition metal dichalcogenide on at least a portion of the first side of the growth substrate; and removing the source substrate from the growth substrate having the layer of the transition metal dichalcogenide thereon.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Section 111(a) application relating to and claiming the benefit of commonly owned, co-pending U.S. Provisional Patent Application No. 62/183,331, titled “DIRECT AND PRE-PATTERNED SYNTHESIS OF TWO-DIMENSIONAL HETEROSTRUCTURES,” having a filing date of Jun. 23, 2015, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The exemplary embodiments relate generally to synthesis of two-dimensional (“2D”) transition metal dichalcogenides (“TMDs”) and, more specifically, to synthesis of TMDs using contact between a thin-film transition metal oxide deposited substrate and a growth substrate for growth. 
     BACKGROUND OF THE INVENTION 
     TMD monolayers are atomically thin semiconductors having direct bandgaps. These monolayers are useful for various types of semiconductors such as photodetectors, optical modulators, solar cells, light-emitting diodes (“LED”), flexible displays, transparent displays, etc. Although there are various useful applications for monolayer and heterostructure TMDs, current chemical vapor deposition methods cannot be used for the growth of large TMDs or over other TMD layers. 
     Typical current techniques for TMD heterostructure fabrication use a transfer process. According to such a process, each TMD monolayer is grown separately and one monolayer is taken off of the growth substrate and stacked on the other monolayer. However, this transfer process is time-consuming, requires alignment when one monolayer is stacked onto another (which is problematic for some optoelectronics applications), and involves the use of polymers which contaminate the interfaces of resulting TMD heterostructures. While techniques for direct growth of heterostructures exist, such techniques have limitations in terms of their achievable size of heterostructures. Furthermore, only bilayered heterostructures have been demonstrated because such techniques cannot add another layer atop bilayered heterostructures since the materials are either evaporated or damaged during the growth of a subsequent layer. 
     SUMMARY OF THE INVENTION 
     The exemplary embodiments relate to a “contact-growth method” for growth of large-area heterostructures as well as growth of TMD monolayers on patterned or unpatterned substrates. Large scale growth and large grain size TMD monolayer growth is essential for further application research. In addition to the large area growth, patterned growth and heterostructure synthesis of TMDs is important for various applications of TMD materials. The exemplary embodiments enable the direct epitaxial of TMD monolayers on other van der Waals materials (e.g., graphene or other TMDs) over a large area (e.g., on the order of over 100 microns), which can be extended to growth of TMD layers on patterned surfaces as well as growth of differing TMDs and other 2D materials on a same substrate. 
     In an embodiment, a method for growing a transition metal dichalcogenide on a substrate includes providing a growth substrate having a first side and a second side opposite the first side of the growth substrate; providing a source substrate having a first side and a second side opposite the first side of the source substrate; depositing a transition metal oxide on at least a portion of the first side of the source substrate; combining the growth substrate with the source substrate such that the first side of the growth substrate contacts the transition metal oxide, the combining producing a substrate stack; exposing the substrate stack to a chalcogenide gas, whereby the transition metal oxide reacts with the chalcogenide gas to produce a layer of a transition metal dichalcogenide on at least a portion of the first side of the growth substrate; and removing the source substrate from the growth substrate having the layer of the transition metal dichalcogenide thereon. 
     In an embodiment, the transition metal oxide is selected from the group consisting of molybdenum trioxide and tungsten trioxide. In an embodiment, the chalcogenide gas includes a chalcogen selected from the group consisting of sulfur and selenium. In an embodiment, each of the growth substrate and the source substrate includes silicon. In an embodiment, the first side of the growth substrate includes an oxidized silicon dioxide layer. 
     In an embodiment, the method also includes the step of forming a pattern on the first side of the growth substrate. The step of forming a pattern is performed between the steps of providing the growth substrate and of combining the growth substrate with the source substrate. The pattern is positioned such that, when the growth substrate is combined with the source substrate, only the pattern on the first side of the growth substrate contacts the transition metal oxide. During the exposing step, the transition metal oxide reacts with the chalcogenide gas to produce the layer of the transition metal dichalcogenide only on the pattern of the first side of the growth substrate. 
     In an embodiment, the step of forming the pattern on the first side of the growth substrate includes the steps of applying a shadow mask to the first side of the growth substrate to produce a masked growth substrate; exposing the masked growth substrate to oxygen plasma; and removing the shadow mask from the masked substrate to produce the growth substrate with the pattern formed on the first side of the growth substrate. In an embodiment, the shadow mask includes at least one of silicon and copper. 
     In an embodiment, the step of forming the pattern on the first side of the growth substrate includes the steps of applying a photoresist mask to the first side of the growth substrate to produce a masked growth substrate; etching the masked growth substrate by inductively-coupled plasma etching; and removing the photoresist mask from the masked substrate to produce the growth substrate with the pattern formed on the first side of the growth substrate. In an embodiment, the inductively-coupled plasma etching is performed using fluoroform plasma. 
     In an embodiment, the pattern is formed by depositing graphene on a portion of the first side of the growth substrate to produce the growth substrate with the pattern formed on the first side of the growth substrate. 
     In an embodiment, the step of depositing a transition metal oxide on at least a portion of the first side of the source substrate includes the step of forming a pattern of the transition metal oxide on a portion of the first side of the source substrate. The pattern of transition metal oxide on the portion of the first side of the source substrate corresponds to a target area of the first side of the growth substrate. When the substrate stack is exposed to the chalcogenide gas, the pattern of transition metal oxide reacts with the chalcogenide gas to produce the layer of the transition metal dichalcogenide on the target area of the first side of the growth substrate. In an embodiment, the step of forming the pattern of the transition metal oxide on the portion of the first side of the source substrate includes the steps of applying a lift-off mask to the first side of the source substrate to produce a masked source substrate; exposing the masked source substrate to an evaporated transition metal oxide; and removing the lift-off mask from the masked source substrate to produce the pattern of transition metal oxide on the portion of the first side of the source substrate. 
     In an embodiment, the method also includes exposing the growth substrate to oxygen plasma. The step of exposing the growth substrate to oxygen plasma is performed after the step of combining the growth substrate with the source substrate and is performed before the step of exposing the substrate stack to the chalcogenide gas. In an embodiment, when the growth substrate is provided, the growth substrate includes a further layer of a transition metal dichalcogenide on a further portion of the first side of the growth substrate. In an embodiment, the at least a portion of the first side of the growth substrate intersects the further portion of the first side of the growth substrate. 
     In an embodiment, the step of exposing the substrate stack to the chalcogenide gas includes the steps of providing a chalcogenide powder having an evaporation temperature, and heating the chalcogenide powder to the evaporation temperature to produce the chalcogenide gas. In an embodiment, the step of exposing the substrate stack to chalcogenide gas includes the steps of providing a furnace, the furnace having an upstream location and a downstream location; placing the chalcogenide powder at the upstream location of the furnace; placing the substrate stack at the downstream location of the furnace; and heating the furnace to the evaporation temperature of the chalcogenide powder, whereby the chalcogenide powder evaporates to produce the chalcogenide gas, and whereby the furnace causes the chalcogenide gas to flow from the upstream location to the downstream location to expose the substrate stack to the chalcogenide gas. 
     In an embodiment, the method also includes depositing a further transition metal oxide on a further portion of the first side of the source substrate. The step of depositing the further transition metal oxide is performed between the steps of depositing the transition metal oxide on the at least a portion of the first side of the source substrate and of combining the growth substrate with the source substrate. When the substrate stack is exposed to the chalcogenide gas, the further transition metal oxide reacts with the chalcogenide gas to produce a layer of a further transition metal dichalcogenide on a further portion of the growth substrate. In an embodiment, the layer of the transition metal dichalcogenide is a monolayer. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  illustrates a flow chart of a method for deposition of TMDs onto a target substrate; 
         FIG. 2  illustrates the elements involved in performance of the method of  FIG. 1  to deposit a TMD onto a plain substrate at various stages of the method of  FIG. 1 ; 
         FIG. 3  illustrates the elements involved in performance of the method of  FIG. 1  to deposit a TMD onto a patterned substrate at various stages of the method of  FIG. 1 ; 
         FIG. 4  illustrates the elements involved in performance of the method of  FIG. 1  to deposit a TMD onto a substrate having a previously deposited TMD thereon at various stages of the method of  FIG. 1 ; 
         FIG. 5  illustrates a P-N junction device fabricated according to the method of  FIG. 1 ; 
         FIG. 6  illustrates a process for surface treatment and contact between source and target substrates to promote the growth of TMDs over a large target area; 
         FIG. 7A  illustrates the process of  FIG. 6  as applied to the growth of a patterned substrate, the pattern being formed through the application of plasma to a growth substrate through a shadow mask; 
         FIG. 7B  illustrates the process of  FIG. 6  as applied to the growth of a patterned substrate, the pattern being formed through inductively coupled plasma etching of a growth substrate using a photoresist mask; 
         FIG. 7C  illustrates the process of  FIG. 6  as applied to the growth of a patterned substrate, the pattern being formed through liftoff patterning of a transition metal oxide on a source substrate; 
         FIG. 8  illustrates a process for growth of different pre-patterned TMDs on a same substrate; 
         FIG. 9  illustrates a process for growth of TMDs over a large target area on graphene oxide; 
         FIG. 10  shows a sequence of photographs, at progressively increasing magnification levels, of bi-layered TMDs grown on a substrate; and 
         FIG. 11  illustrates the anti-oxidation properties of a substrate having graphene and a tungsten sulfide monolayer grown thereon. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The exemplary embodiments relate to contact-growth techniques for growth of transition metal dichalcogenides (“TMDs”) in various manners. In an embodiment, TMDs are deposited over a large target area. In an embodiment, TMDs are grown on a patterned substrate. 
       FIG. 1  shows an exemplary method  100  for deposition of a TMD onto a substrate. In step  110  of the method  100 , a source substrate is prepared. This step may be accomplished by depositing a thin film of a transition metal oxide onto one side of a substrate. In an embodiment, the transition metal oxide includes molybdenum trioxide (MoO 3 ). In an embodiment, the transition metal oxide includes tungsten trioxide (WO 3 ). In an embodiment, deposition may be performed with an e-beam evaporator. 
     In step  120  of the method  100 , a growth substrate is prepared. In an embodiment, a clean substrate may be prepared for use in large area TMD growth. For oxygen plasma assisted growth, a growth substrate is exposed to plasma directly before growth. Pre-growth oxygen plasma treatment may increase the crystal size of grown monolayer tungsten disulfide (WS 2 ). For example, a growth process performed on an untreated substrate results in a nucleation density of 940±15 crystals/mm 2  and an average WS 2  crystal size of 56±1 μm 2 . With plasma treatment, the nucleation density decreases by 8% to 871±11 crystals/mm 2  while the average WS 2  crystal sized increases by 78% to 101±2 μm 2 . To produce patterned growth, a contact shadow mask is used during the plasma treatment process; the mask is removed before growth, leaving only the surface energy difference to do the patterning. Another growth is run with process parameters optimized for selectivity between the plasma treated and untreated regions. 
     In an embodiment, a patterned substrate may be prepared for patterned TMD growth; in such an embodiment, preparation of the substrate may include etching a desired pattern on the substrate via a conventional lithography or dry-etching process. In an embodiment, a TMD pre-grown substrate may be prepared for TMD heterostructure synthesis. 
     In step  130 , the source substrate and growth substrate, as prepared in steps  110  and  120 , respectively, are stacked with one another. In this step, the side of the source substrate having the transition metal oxide deposited thereon is brought into contact with the target portion (e.g., the pattern of a patterned substrate to be used for patterned TMD growth; the TMD predeposit of a substrate to be used for TMD heterostructure synthesis) of the growth substrate. 
     In step  140 , a furnace is prepared for use in TMD formation. In an embodiment, the furnace is a laboratory tube furnace. In an embodiment, the furnace is an OTF-1200X furnace manufactured by MTI Corporation of Richmond, Calif. However, it will be apparent to those of skill in the art that alternative embodiments may use any type of heat source capable of applying heat as described herein. In this step, a chalcogenide in powder form is placed into a furnace tube upstream of an intended reaction area. In an embodiment, the chalcogenide includes sulfur. In an embodiment, the chalcogenide includes selenium. The stacked substrate formed in the third step is placed in the middle of the furnace tube and a vacuum is generated in the furnace tube. 
     In step  150 , the furnace tube is heated to evaporate the chalcogenide powder. In an embodiment, the furnace tube is heated to 900° C. The evaporation of the chalcogenide powder produces chalcogenide gas. The chalcogenide gas reacts with the transition metal oxide, resulting in the formation of a new TMD monolayer on the growth substrate. In an embodiment wherein the transition metal oxide includes molybdenum trioxide and the chalcogenide includes sulfur, the TMD monolayer includes molybdenum disulfide (MoS 2 ). In an embodiment wherein the transition metal oxide includes molybdenum trioxide and the chalcogenide includes selenium, the TMD monolayer includes molybdenum diselenide (MoSe 2 ). In an embodiment wherein the transition metal oxide includes tungsten trioxide and the chalcogenide includes sulfur, the TMD monolayer includes tungsten disulfide (WS 2 ). In an embodiment, wherein the transition metal oxide includes tungsten trioxide and the chalcogenide includes selenium, the TMD monolayer includes tungsten diselenide (WSe 2 ). In an embodiment, the TMD monolayer may be appropriate for use in a p-type semiconductor. In an embodiment, the TMD monolayer may be appropriate for use in an n-type semiconductor. 
     In step  160 , the source substrate is removed from the growth substrate with the TMD monolayer formed thereon. More particularly, following the formation of the TMD monolayer, as described above, the source substrate does not adhere to the growth substrate with the TMD monolayer formed thereon. Therefore, the source substrate may easily be removed therefrom. 
     In the method  100 , the growth and source substrates may be of any size that may be accommodated in the furnace used for the reaction; thus, TMDs may be grown over a large target area. Additionally, because the target substrate may be patterned in any desired manner, TMD growth may be targeted to the patterned area. Further, because an existing TMD deposit may exist on the growth substrate, a further TMD monolayer may be added to the existing TMD deposit without requiring any transfer of one or more TMD layers to be performed. 
       FIG. 2  illustrates the elements existing at various stages of the performance of the method of  FIG. 1  when a TMD is grown on a plain substrate. At a first stage  210 , a thin film  212  of a transition metal oxide (e.g., MoO 3 , WO 3 , etc.) is coated on a source substrate  214 , which is shown both from the top and from the side. At a second stage  220 , a plain (e.g., not patterned) growth substrate  222  is provided; the growth substrate  222  is also shown from the top and from the side. At a third stage  230 , shown from the side, the source and growth substrates are placed together such that the transition metal oxide thin film  212  is sandwiched between the source substrate  214  and the growth substrate  222 . The sandwiched combination of substrates is processed using an oven, as described above with reference to steps  140  and  150  of the method  100 . Once processed, in a final stage  240 , also shown from the side, a TMD monolayer  242  (e.g., MoSe 2 , WS 2 , etc.) remains on the growth substrate  222  after the source substrate  214  has been removed therefrom. 
       FIG. 3  illustrates the elements existing at various stages of the performance of the method of  FIG. 1  when a TMD is grown on a patterned substrate. At a first stage  310 , a thin film  212  of a transition metal oxide (e.g., MoO 3 , WO 3 , etc.) is coated on a source substrate  214 , which is shown both from the top and from the side, and which is identical to the source substrate  214  described above with reference to  FIG. 2 . At a second stage  320 , a growth substrate  322  having a pattern  324  formed therein is provided; the growth substrate  322  is also shown from the top and from the side. At a third stage  330 , shown from the side, the source and growth substrates are placed together such that the transition metal oxide thin film  212  is sandwiched between the source substrate  214  and the growth substrate  322 . The sandwiched combination of substrates is processed using an oven, as described above with reference to steps  140  and  150  of the method  100 . Once processed, in a final stage  340 , also shown from the side, a patterned TMD monolayer  342  (e.g., MoSe 2 , WS 2 , etc.) remains on the growth substrate  322  after the source substrate  214  has been removed therefrom. 
       FIG. 4  illustrates the elements existing at various stages of the performance of the method of  FIG. 1  when a TMD is grown on a substrate that has a previously—deposited TMD layer thereon. At a first stage  410 , a thin film  412  of a transition metal oxide (e.g., MoO 3 , WO 3 , etc.) is coated on a desired portion of a source substrate  414 , which is shown both from the top and from the side. At a second stage  420 , a growth substrate  422  having a TMD layer  424  deposited thereon is provided; the growth substrate  422  is also shown from the top and from the side. At a third stage  430 , shown from the side, the source and growth substrates are placed together such that the transition metal oxide thin film  412  and the TMD layer  424  are sandwiched between the source substrate  414  and the growth substrate  422 . The sandwiched combination of substrates is processed using an oven, as described above with reference to steps  140  and  150  of the method  100 . Once processed, in a final stage  440 , also shown from the side, a patterned TMD monolayer  442  (e.g., MoSe 2 , WS 2 , etc.) remains on the growth substrate  422  and overlays the TMD layer  424  after the source substrate  414  has been removed therefrom. 
       FIG. 5  illustrates a P-N junction device  500  fabricated in accordance with the process described above with reference to  FIG. 4 . A silicon dioxide (SiO 2 ) substrate  510  has a first TMD layer  520  (e.g., a layer of WSe 2 ) grown thereon and extending between two palladium terminals  522 ,  524 . A second TMD layer  530  (e.g., a layer of MoS 2 ) is subsequently grown thereon and extends between two aluminum terminals  532 ,  534 . A P-N junction area  540  is formed at the intersection of the first and second TMD layers  520 ,  530 . 
       FIG. 6  illustrates a process  600  for preparing and joining substrates to grow a TMD layer over a large target area. A prepared source substrate  610  includes a layer  612  of a transition metal oxide (e.g., WO 3 ) deposited on a silicon substrate  616  having an oxidized silicon dioxide surface  614 . A prepared growth substrate  620  includes an oxidized silicon dioxide surface  622  of a silicon substrate  624 . The growth substrate  620  is exposed to oxygen plasma  626  directly before growth, which leads to increased crystal size as described above. The prepared source substrate  610  and prepared growth substrate  620  may be combined into a sandwich  630 , which may then be placed into a furnace  640  as described above. A chalcogenide powder  660  (e.g., sulfur, selenium) form is placed into a furnace tube upstream of an intended reaction area, and may be induced to flow through the furnace  640  by a flow of gas (e.g., argon and/or hydrogen). As described above, as a result of the process  600 , a TMD is grown on the growth substrate  620 . 
       FIGS. 7A-7C  illustrate processes for patterned TMD growth. In the process  700  shown in  FIG. 7A , a source substrate  710  with a transition metal oxide layer  712  is provided. A growth substrate  720  is also provided. The growth substrate  720  is treated with oxygen plasma through a shadow mask (e.g., made from both polished silicon and rolled copper foils), resulting in a pattern  722  being formed on the growth substrate  720 . The source substrate  710  and the treated growth substrate  720  are combined to form a sandwich  724 . The sandwich  724  is then processed using a furnace  750 , as described above, to form a patterned TMD layer on the growth substrate  720 . 
     In the process  702  shown in  FIG. 7B , a source substrate  710  with a transition metal oxide layer  712  is provided. A growth substrate  720  is also provided. A photoresist mask  730  is applied to the growth substrate  720  and a pattern  732  is etched using inductively-coupled plasma (“ICP”) etching through the use of CHF 3  plasma. The photoresist mask  730  is then removed using acetone (e.g., 55 nm deep in a 90 nm oxide). The source substrate  710  and the etched growth substrate  720  are combined to form a sandwich  734 , which is subjected to a pre-growth oxygen plasma treatment. The sandwich  734  is then processed using a furnace  750 , as described above, to form a patterned TMD layer on the growth substrate  720 . 
     In the process  704  shown in  FIG. 7C , a source substrate  710  with a transition metal oxide layer  712  is provided. A growth substrate  720  is also provided. A pattern  740  is formed in the transition metal oxide layer  712  using liftoff and electron beam evaporation of 50 nm WO 3  from pellets. The patterned source substrate  720  and the growth substrate  720  are combined to form a sandwich  742 , which is subjected to a pre-growth oxygen plasma treatment. The sandwich  742  is then processed using a furnace  750 , as described above, to form a patterned TMD layer on the growth substrate  720 . 
       FIG. 8  illustrates a process  800  for the growth of disparate pre-patterned TMD layers on a same substrate. In a first stage of the process, a pre-patterned thin film  812  of a first transition metal oxide (e.g., MoO3) is coated onto a substrate  810  (e.g., a silicon substrate). In a second stage of the process, a pre-patterned thin film  814  of a second transition metal oxide (e.g., WO 3 ) is coated onto the substrate  810 . In a third stage of the process, the patterned substrate  810  is sandwiched with a growth substrate (e.g., a silicon substrate  820  with a silicon dioxide layer  822  thereon). The sandwiched substrates are processed using an oven, as described above with reference to steps  140  and  150  of the method  100 . As a result, two differently patterned TMD layers  832  (e.g., a MoS 2  layer) and  834  (e.g., a WS 2  layer) are grown on the substrate  820 . 
       FIG. 9  illustrates a process  900  for the growth of TMD layers on graphene. In a first stage of the process, a growth substrate (e.g., a silicon substrate  910  with a silicon dioxide layer  912 ) with patterned graphene  914  thereon is provided. In a second stage of the process, a source substrate  920  having a transition metal oxide  922  (e.g., WO 3 ) patterned thereon is provided, and the substrates  910  and  920  are sandwiched together as described above. The substrates are processed using an oven, as described above with reference to steps  140  and  150  of the method  100 . As a result, the substrate  910  has a patterned TMD layer  930  deposited on the patterned graphene  914 . 
       FIG. 10  illustrates sequentially magnified photographs  1000 ,  1002 ,  1004  showing bi-layered TMDs deposited on a substrate. In the photograph  1000 , a substrate  1010  (e.g., an oxidized silicon wafer) is shown having an array of locally grown TMDs  1012  thereon. The photograph  1000  includes a magnification indicator  1014  indicating the portion of the photograph  1000  that is shown in the magnified photograph  1002 . In the photograph  1002 , the substrate  1010  is shown in greater detail. The locally grown TMDs  1012  can be seen more clearly. The photograph  1002  includes an array of circles  1020 , which outline the locations of patterned transition metal (e.g., MoO 3 , WO 3 , etc.) applied to the substrate  1010  to produce the TMDs  1012 . In an embodiment, the patterned transition metal may be applied to the substrate through the use of a patterned photoresist layer. The photograph  1002  includes a magnification indicator  1022  indicating the portion of the photograph  1002  that is shown in the further-magnified photograph  1004 . The photograph  1004  shows, in greater detail, the locally grown TMDs  1012 . The photograph  1004  includes triangles  1030 , which indicate the borders of single crystalline monolayer islands. The photograph  1004  also shows bi-layered TMDs  1032 , which are grown on top of monolayered TMDs. 
     In an embodiment, TMD growth as described above may be performed during the fabrication of field effect transistors (“FET”). In an embodiment, TMD deposition as described above may be performed during the fabrication of optoelectronic devices. TMD monolayers are direct band gap semiconductors, which makes them react strongly with light. TMD monolayers also have high absorption coefficients and efficient electron-hole pair generation. These properties suggest TMDs are good candidates for photodetectors and optical modulators. Furthermore, the direct band gap of TMD is useful in the fabrication of light emitting diodes (“LEDs”). 
     In an embodiment, TMD growth as described above may be performed during the fabrication of flexible and stretchable electronic components for use in wearable devices. TMD monolayers are atomically thin (e.g., on the order of about 1 nanometer) and have a failure strain of 11% to 25%. The thinness of TMDs makes them flexible out of plane. The combination of desirable mechanical properties (e.g., capability to be flexed and stretched) and electrical properties make TMD monolayers a useful material for wearable devices. 
     In an embodiment, TMD growth as described above may be performed during the fabrication of solar cells. TMD monolayers can absorb 5% to 10% of incident light despite their thinness, which is more than double the absorption rate of graphene. The high absorption rate, efficient electron-hole generation, fast relaxation time, and type II energy band alignment of TMD heterostructures make them a good material for solar cells. The generated power per unit volume of a solar cell using TMD heterostructures may be higher by a factor of about 10 than that of a gallium arsenide solar cell. 
     The exemplary embodiments for contact growth of TMDs enable fabrication of heterostructures, facilitating “anti-degradation.” For example, the exemplary embodiments may provide for anti-oxidation of WS 2  on graphene. Suspended WS 2 /graphene does not exhibit oxidation in ambient air, which may be attributed to a lack of defects and local electric-fields. Oxidation of WS 2  occurs at localized areas containing defects, such as edges or grain boundaries, and oxidation of the interior of the perfect single crystalline WS 2  occurs due to the rough surface of the SiO 2  substrate. However, no oxidation occurs in the interior of WS 2  when it is grown on graphene because graphene screens the existing defects of the SiO 2  substrate, diminishing potential initiation sites for oxidation. 
       FIG. 11  is an image  1100  of a substrate  1110  having a spike  1120  protruding therefrom; it will be apparent to those of skill in the art that such spikes may commonly be found on substrates having an oxidized silicon surface. If a TMD is grown on such a surface, in accordance with the processes described above, the TMD microlayer may be non-uniform, which may lead to oxidation. Additionally, spikes such as the spike  1120  may lead to the induction of local electric fields, which may also lead to oxidation. However, in  FIG. 11 , a graphene layer  1130  has been deposited onto the substrate  1110  prior to TMD growth. The presence of the graphene layer  1130  covers the spike  1120 , thereby smoothing the surface of the substrate  1110  prior to TMD growth and reducing the effect of electric fields. A TMD layer  1140  has been grown atop the graphene layer  1130  as described above with reference to  FIG. 9 . Due to the presence of the graphene layer  1130  on the substrate and below the TMD layer  1140 , oxidation of the TMD layer  1140  may be extremely slow or even non-existent. 
     It should be understood that the embodiments described herein are merely exemplary in nature and that a person skilled in the art may make many variations and modifications thereto without departing from the scope of the present invention. All such variations and modifications, including those discussed above, are intended to be included within the scope of the invention exemplified in the attached claims.