Room Temperature van der Waals Epitaxy of Metal Thin Films on

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Room Temperature van der Waals Epitaxy of Metal Thin Films on Molybdenum Disulfide Anna Domask, Kayla A. Cooley, Bernd Kabius, Michael Abraham, and Suzanne E. Mohney Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00257 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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Crystal Growth & Design

Room Temperature van der Waals Epitaxy of Metal Thin Films on Molybdenum Disulfide Anna C. Domask,† Kayla A. Cooley,† Bernd Kabius,‡ Michael Abraham,† and Suzanne E. Mohney∗,† †Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA ‡Materials Research Institute, The Pennsylvania State University, University Park, PA E-mail: [email protected] Phone: +1 814-863-0744

Abstract Transmission electron microscopy, in particular selected area electron diffraction, was used to investigate the orientational relationship of Al, Ag, Cu, Mn, Mo, Ni, Pd, Ru, Re, and Zn deposited via physical vapor deposition on MoS2 at room temperature. Past work has shown that a few face-centered cubic (FCC) metals (Ag, Au, Pb, Pd, and Pt) could be deposited epitaxially on MoS2 . However, we found that additional FCC metals (Al and Cu) could be deposited epitaxially at room temperature on MoS2 with the orientational relationship M(111)kMoS2 (0001) and M[2¯20]kMoS2 [11¯20], while a hexagonally close packed (HCP) metal Zn was epitaxial on MoS2 with a M(0001)kMoS2 (0001) and M[11¯ 20]kMoS2 [11¯20] relationship. On the other hand, the FCC metal Ni, body-centered cubic metal Mo, and HCP metals Re and Ru were not epitaxial on deposition or even after annealing at 673 K for 4 h. By comparing the results with both physical constants and modeling of the metal/MoS2 systems, we

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observed that metals with a close-packed plane with six-fold symmetry, a high homologous temperature, and a low barrier to surface diffusion on MoS2 are more likely to grow epitaxially at room temperature on MoS2 .

Introduction Essential to many efficient uses of MoS2 and other TMDs for many applications is an understanding of how metals react and/or interact with the few-layer transition metal dichalcogenide (TMD). A lack of low-resistance contacts to these semiconductors is an obstacle to the performance of TMD-based electronic devices. 1 Since bonding and disorder at the interface impact the contact resistance, the metal/TMD interface must be better understood. There are a few possible results when a TMD and metal are placed into contact: they can react to form new compounds, metal atoms can substitute on the Mo or S lattices, metal atoms can intercalate between the layers of the TMD, or the two materials can have an atomically abrupt interface. Prior work indicates that the formation of new compounds are thermodynamically favored to occur at room temperature in many early transition metalMoS2 systems. 2 However, a purely thermodynamical view ignores the equally important role that kinetics plays in real systems. Therefore, experimentation is necessary to determine whether transition metals react with MoS2 at moderate temperatures and times typically used in device processing and packaging and to reveal other features of these heterostructures. A number of metals are known to grow epitaxially on MoS2 , and more were discovered in this work. For many of them, additional thermal energy from annealing slightly changes the perfection of the epitaxy but does not allow for epitaxial reorganization of any metal that was not epitaxial on deposition. The epitaxy is conveniently characterized using planview transmission electron microscopy (TEM), particularly selected area electron diffraction (SAED). Single crystal metal may be a good substrate (and even a patternable template 3 ) for the growth of a TMD layer. On the other hand, deposition of MoS2 on polycrystalline Au resulted 2


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in (0001)MoS2 with different rotations, 4 while the grain boundaries served as nucleation points for MoS2 growth. Single crystal metal might also provide a route for the direct transformation of that metal to its corresponding TMD through “soft” chalcogenization or solid state reaction. One possible application of a semiconductor/metal/semiconductor heterostructure is a high-frequency metal-base transistor, where the metal forms the base and the semiconductors serve as the emitter and collector. 5 For optimum device performance, the metal and semiconductors should be epitaxial, as occurs for Si/CoSi2 /Si. 6 Epitaxial metals on TMDs are also interesting because the precise arrangement of atoms across an interface can impact the Schottky barrier height of the interface for the same metal/semiconductor pair by more than a third of the band gap of the semiconductor. 7–9 It is well known that contacts to TMDs exhibit partial Fermi level pinning; the barrier height is neither independent of the metal selection nor due only to the difference between the work function of the metal and the electron affinity of the semiconductor. 10 Work by Gong et al. attempted to explain the partially pinned Fermi level seen in metal/MoS2 contacts, but their work did not include the effects of epitaxial arrangement. 11 Whether epitaxy alters the Schottky barrier height for metal/van der Waals (vdW) epitaxial systems is not yet known, but the first step is identifying epitaxial metal/TMD systems with which to study the effect. Typically, epitaxy requires that the substrate and deposited film have a plane with the same or related symmetry and a very small lattice mismatch, 12 or else the lattice mismatch can be accommodated by a large number of misfit dislocations at the film/substrate interface. 13 Van der Waals epitaxy, due to the lack of covalent bonding across the van der Waals gap, allows for even larger lattice mismatches of up to 50% without the need for large numbers of misfit dislocations. 14–16 Given that there is no strong bonding across the van der Waals gap, one might assume that all vdW solids and metal combinations behave the same way, but this is not the case. 14 Metal/MoS2 epitaxy has been previously observed predominantly in the late transition metals, specifically Ag, 17–20 Au, 21–23 Pb, 24 Pd, 22,25 and Pt. 26 All of these metals orient with

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Figure 1: Schematic of the plane with six-fold symmetry and the formula for calculating the atomic spacing for the a) FCC, b) HCP, and c) BCC crystal structures. the M(111)kMoS2 (0001); M[110]kMoS2 [11¯20] epitaxial relationship, where M denotes the metal. It should be noted that all previously reported metals that are epitaxial on MoS2 are FCC. Only one epitaxial relationship between FCC metals and MoS2 has been experimentally observed in continuous films (after coalescence). Despite having a smaller lattice mismatch, the “rotated” {111} pattern is not seen experimentally in continuous films because it is not energetically favored. 27 While many epitaxial relationships between FCC metals and MoS2 can be imagined, plan-view TEM (used extensively in this study) can distinguish between them. Table 1 contains structural information for all the metals considered in this paper at room temperature and atmospheric pressure. Prototype structure and space group indicate atomic arrangement, while a and c indicate the lattice parameters. The atomic distance is the interatomic distance in the lowest index plane that exhibits six-fold symmetry (the FCC(111), BCC(111), and HCP(0001)). Note that the interatomic distance in the case of BCC(111) is not the shortest distance between atoms. Figure 1 shows a schematic of the planes and lattice spacing calculation for each crystal structure: a) FCC, b) HCP, and c) BCC. Shown in Table 1, the MoS2 lattice mismatch compares the calculated atomic distance within the metal planes of interest with the lattice parameter of MoS2 . All raw data is from the CRC Handbook. 28 The lattice mismatch between MoS2 and the metals known to be

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epitaxial on MoS2 is all greater than 9%, lending credence to the idea that van der Waals epitaxy does not require the same small lattice mismatch that ordinary epitaxy does. 16 Table 1: Structure of metallic elements at room temperature and pressure. a and c indicate the lattice parameters. The lattice mismatch compares the calculated atomic distances on the (111) plane of FCC metals, the (111) plane of BCC metals, and the (0001) plane of HCP metals with the a lattice parameter of MoS2 . All raw data is from the CRC Handbook. 28

Metal

αAl Ni Cu Pd Ag Pt Au Pb Mo αMn Zn Ru Re

Prototypical Space Structure Group

Cu Cu Cu Cu Cu Cu Cu Cu W αMn Mg Mg Mg

Fm3m Fm3m Fm3m Fm3m Fm3m Fm3m Fm3m Fm3m Im3m I43m P63/mm P63/mm P63/mm

a

c

Atomic Distance

(nm)

(nm)

(nm)

0.40496 0.3524 0.36146 0.38903 0.40857 0.39236 0.40782 0.49502 0.3147 0.89126 0.2665 0.4947 0.27058 0.42816 0.27609 0.4458

Lattice Mismatch

0.28635 0.24918 0.25559 0.27509 0.28890 0.27744 0.28837 0.35003 0.44505

10.4% 26.9% 23.7% 14.9% 9.4% 13.9% 9.6% −9.7% −29.0%

0.2665 0.27058 0.27609

18.6% 16.8% 14.5%

Methods The overall procedure was 1) physical exfoliation of MoS2 onto TEM grids, 2) physical vapor deposition of metals, 3) annealing (of some samples), and 4) plan-view TEM microscopy. R holey carbon TEM grids was performed using Exfoliation of MoS2 onto Quantifoil

blue Nitto thermal release tape. The MoS2 was purchased as a lab-grown, bulk crystal from 2-D Semiconductors, Inc. Physically exfoliated, bulk lab-grown MoS2 was used because it typically has the lowest defect density of any few-layer MoS2 substrate. 29 All the metals, except Mn, were deposited via UHV DC magnetron sputtering. The base pressure was less than 10−7 Torr and depositions were preformed using 5 mTorr ultra-high 5



purity (UHP) Ar as the sputtering gas at a rate of at 1.0 ˚ A/s. Metal thickness was constantly monitored with a crystal monitor for all samples except Ni. (The metal deposition rate and ultimate thickness of the Ni sample was calibrated via a preliminary calibration sample under the same sputtering conditions.) The final metal thickness was 15 nm for the Ag and Pd samples and 30 nm for the Al, Ru, Re, Zn, Ni, and Cu samples. The Mn was evaporated via electron-beam evaporation. The base pressure was less than 10−8 Torr and deposition was performed at 1.0 ˚ A/s. The deposited metal thickness was determined and constantly measured with a crystal monitor to a final thickness of 30 nm. Some samples were annealed using either a rapid thermal annealing (RTA) system or a tube furnace, depending on annealing duration. RTA annealing was performed in Ar gas with a flow rate of 3 slpm that had been gettered over Ti sponge. The furnace was heated to the intended temperature at a rate of 75 K/s and samples were annealed for 5 minutes. RTA annealing was later discontinued for most samples in favor of tube furnace annealing so the anneals could be hours long instead of minutes, to allow the samples more chance to achieve thermodynamic equilibrium. Most samples were annealed in a tube furnace for 4 h at various temperatures. The samples were annealed in UHP Ar flowing at a rate of 100 sccm. The Ar was gettered using Zr to remove remove additional oxygen. TEM microscopy was performed in plan view at 80 kV to minimize knock-out damage. This work was done predominantly with a FEI Talos F200X, although some early work was performed in a FEI Tecnai 200 kV D2315 XTwin. Selected area electron diffraction (SAED) was performed to determine the crystal structure and orientation of samples. Energy dispersive spectroscopy (EDS) via the FEI Talos F200X was used to confirm the presence of a MoS2 flake and to monitor contamination such as oxidization. One cross-section was made using a FEI Helios Nanolab 660 focused ion beam. Crosssectional images of these interfaces were made and imaged using high-resolution TEM and STEM using a FEI Titan G2 aberration corrected microscope at 300 kV. High angle annular dark field scanning transmission electron (HAADF-STEM) was used to collect high

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Z-contrast images.

Results A number of metals were found in this study to be epitaxial on MoS2 including Ag, Pd, Al, Zn, and Cu. Alternatively, Mn, Mo, Ru, Re, and Zn were not epitaxial, although some ordering of small grains was seen in Re. No metal tested was found to transform to reaction products upon annealing to 673 K.

Face-Centered Cubic Metals Silver Ag is epitaxial on MoS2 with a ± 2◦ misalignment (spread) upon deposition. That misalignment disappears upon annealing to a moderate temperature (473 K). The epitaxial orientation is Ag(111)kMoS2 (0001); Ag[2¯20]kMoS2 [11¯20]. This relationship is consistent with what is reported in the literature. 18 The SAED as viewed in plan-view TEM of Ag on MoS2 on deposition can be seen in Figure 2 a). Double diffraction of Ag and MoS2 is present and only the primary Ag spots are labeled (in italics). Also, the Ag spots are not discrete because there is a ± 2◦ rotational misalignment. The SAED of a sample after 4 h at 573 K can be seen in Figure 2 b) and an ADF-STEM image of this flake can be seen in c). Double diffraction of Ag and MoS2 can clearly be seen, resulting in multiple Ag spots (labeled in italics) surrounding each brighter MoS2 spot (labeled upright). The SAED shows no misalignment after annealing at a temperature of 473 K or higher, in agreement with the literature, 19 which found no misalignment with a slow deposition rate or deposition onto a substrate at an elevated temperature. In all cases, the Ag de-wetted and agglomerated on the MoS2 (as well as on the carbon background) to some degree, in agreement with the literature. 30 For instance, Figure 2 c) clearly shows the Ag (in the top right) agglomerated into a ball. The Ag island size was 7



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Figure 2: a) SAED of Ag on MoS2 on deposition showing an epitaxial relationship of Ag(111)kMoS2 (0001); Ag[2¯20]kMoS2 [11¯20]. The Ag has a ± 2◦ rotational misalignment, which can be seen where the spots have spread. b) SAED of Ag on MoS2 after 4 h at 573 K showing the same epitaxy but without the misalignment. c) ADF-STEM micrograph of the same sample. larger on the MoS2 than on the amorphous carbon grid. Particularly after annealing at 473 K or higher, Ag could be seen diffusing on the surface and coalescing into larger Ag islands. The Ag island size was larger on the MoS2 than on the amorphous carbon grid. The continued evolution of agglomerated Ag on MoS2 has been reported previously. 23 van der Waals epitaxial materials can exhibit strong clustering of the deposited metal and therefore a Volmer-Weber growth mode. 14 The growth mode can be predicted using the ratio Eads /Ecoh from Saidi. 31 Those values for Ag would predict a Volmer-Weber growth mode for Ag, which can clearly be seen in Figure 2 c). Agglomeration was observed in some other epitaxial metals, but most notably in Ag. Figure 3 shows an atomic resolution image of a cross-section of an Ag/MoS2 contact that has been annealed at 573 K for 5 min. The image is a HAADF-STEM micrograph that was imaged at 300 kV. The two differences between this sample and our plan-view samples was the duration of annealing (5 min vs. 4 h) at 573 K and the Ag was capped with 10 nm of electron beam deposited SiO2 . The SiO2 cap inhibited the agglomeration of Ag on MoS2 that was seen in our plan-view samples. The exact nature of the bonding between Ag and MoS2 was not determined, but no new phases formed.

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[111] [022]

Ag MoS2 SiO2 Figure 3: Atomic resolution HAADF-STEM performed at 300 kV of an Ag/MoS2 contact annealed at 573 K for 5 min. The electronic characteristics of this interface are reported in another paper by our group. 32 On annealing, the contact resistance decreased. This improvement may have been due to doping by Ag donors, as will be explored in more detail in a future publication. Also, annealing likely improved the epitaxy since our work on room-temperature deposition showed a misalignment angle and this image of an annealed sample does not. Finally, the epitaxial Ag on the MoS2 shows no line defects or grain boundaries within the field of view.

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Figure 4: a) SAED of Al on MoS2 on deposition with an epitaxial relationship of Al(111)kMoS2 (0001); Al[2¯20]kMoS2 [11¯20]. The Al is single crystalline, but there is some misalignment, as can be seen in the spread of the spots where they almost form a hexagon around the MoS2 spots. b) SAED of a MoS2 flake with Al atop it that has been annealed for 4 h at 673 K, with the same epitaxial relationship, but no misalignment. The indexes are labeled, with MoS2 in upright numbers and Al in italics. Aluminum Al was epitaxial on MoS2 upon deposition with a relationship of Al(111)kMoS2 (0001); Al[2¯20]kMoS2 [11¯20]. The SAED can be seen in Figure 4 a). Upon deposition, both double diffraction of Al and MoS2 and a 2◦ misalignment within the Al plane of the sample were present. These two effects created an almost complete hexagon around the MoS2 spots. Upon annealing for 4 h at 673 K, the resulting orientations is still Al(111)kMoS2 (0001); Al[2¯20]kMoS2 [11¯20]. Double diffraction is still present, but the angular misalignment present before annealing has disappeared. This change can be seen in Figure 4 b).

Copper Copper is epitaxial on deposition with an arrangement of Cu(111)kMoS2 (0001); Cu[2¯20]kMoS2 [11¯20] and a small misalignment angle of ±2◦ . Figure 5 shows a SAED pattern of the epitaxy in-

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220 1010 1120 0110 202

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Figure 5: SAED of a 30 nm Cu thin film on MoS2 on deposition. The Cu is epitaxial with a relationship of Cu(111)kMoS2 (0001); Cu[2¯20]kMoS2 [11¯20] and a ±2◦ misalignment angle or spread. cluding double diffraction. All attempts to investigate Cu on MoS2 after 4 h at 673 K resulted in oxidization. Nickel Ni has a moderate melting temperature (1728 K) 28 and a low mobility and high barrier to diffusion on MoS2 . 31 This situation is in contrast to many of the other FCC metals that have higher mobility and lower barriers to diffusion on MoS2 . On deposition, 30 nm of Ni was polycrystalline with fine grains on MoS2 . There is also no preferred orientation. Figure 6 a) shows the SAED pattern with clear double diffraction of the polycrystalline rings around the MoS2 spots. The results for Ni on MoS2 after 5 min at 673 K can be seen in Figure 6 with the SAED in b). The results are very similar to the results on deposition. The Ni is still polycrystalline and has no ordering, although the double diffraction is less visible. The grain size has increased somewhat as can be seen from the spottier rings in the SAED pattern. Also, the

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Figure 6: a) SAED of a 30 nm Ni thin film on MoS2 on deposition. b) SAED of a 30 nm Ni thin film on MoS2 after 5 min at 673 K. Ni was not significantly oxidized. Ni is not epitaxial on MoS2 . This Ni/MoS2 sample was annealed for only 5 min at 673 K in Ar using an RTA because after annealing for 4 h at 673 K in Ar in a tube furnace, all of the Ni on the surface was oxidized. This oxidization made it impossible to evaluate the orientational relationship between Ni and MoS2 . On annealing to 673 K for 4 h, a fraction of the Ni that had been atop the MoS2 (presumably before oxidation was complete) might have intercalated into the van der Waals spaces, while the remainder oxidized to form NiO. The indiffusion was observed by Auger electron spectroscopy and Raman spectroscopy and will be detailed in a future article. Indiffusion has previously been observed in the Ni/MoS2 system, but at a much higher temperature (1200 K). 33 Any possible intercalation of Ni was not detected in plan-view SAED even upon tilting the flake because any expansion of the lattice in the c-direction was less than the resolution of SAED. Also, if Ni had intercalated and additionally ordered, new diffraction spots might have been visible when the sample was tilted, but such spots were not seen. 34

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Palladium This study agreed with the literature that Pd is epitaxial on MoS2 upon deposition. 22 The epitaxy is described as Pd(111)kMoS2 (0001); Pd[2¯20]kMoS2 [11¯20]. There was no significant misalignment upon deposition. There was also no reaction after annealing to 573 K for 5 min.

Body-Centered Cubic Metals Manganese Manganese is a BCC metal with over 50 atoms in its basis. Mn presented some experimental challenges; despite using UHP Ar and a Zr getter, every annealing attempt resulted in oxidization of the Mn. We were, on the other hand, able to determine what occurs when Mn is deposited on MoS2 at room temperature. Mn is not epitaxial on MoS2 on deposition. Additionally, it does not react upon deposition at room temperature and is polycrystalline. The SAED of the sample can be seen in Figure 7. The only visible Mn ring is the Mn 114, which is the ring that normally has the strongest diffraction. Additionally, the 114 ring can be seen, more faintly, around each of the low-index MoS2 spots due to double diffraction. In the as-deposited samples, the EDS indicated that the Mn surrounding the MoS2 on the carbon grid was partially oxidized. Manganese oxide compound rings were also indexed in the SAED of the background, but the Mn on the flake did not substantially oxidize. On the other hand, there is a lack of reflections from any manganese oxide rings in the SAED in Figure 7 obtained from the Mn/MoS2 interface. Molybdenum Molybdenum on MoS2 was similar to Mn: Mo did not react and was polycrystalline with double diffraction rings around the low index MoS2 spots. All of the BCC polycrystalline

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114 1010 1120 0110

4 1/nm Figure 7: SAED of a MoS2 flake with Mn atop. The Mn 114 polycrystalline diffraction ring centered on the direct beam as well as the low-index MoS2 spots. The indices are labeled, with MoS2 in upright numbers and Mn in italics. rings for Mo were detected, indicating a lack of texturing and a lack of transformation to FCC Mo, as has been reported upon sputtering Mo onto mica and rocksalt. 35 The SAED of a representative sample can be seen in Figure 8 a). No epitaxy or reaction occurred on deposition. Upon annealing for 4 h at 673 K, Mo did not oxidize, but the diffraction pattern did not change on annealing and no reaction was detected. The SAED of a representative sample can be seen in Figure 8 b).

Hexagonal Close Packed Metals Rhenium On deposition, Re lacks the single crystal epitaxy previously seen in many of the FCC metals. Some individual small grains can be indexed to various orientational relationships, but most of the flake lacks a single epitaxial relationship or cannot be indexed. In all cases, Re exhibited an HCP structure (and not an FCC structure, as has been reported by Chopra 14


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Figure 8: a) SAED of a MoS2 flake with Mo atop it. The polycrystalline rings of Mo are labeled. Mo rings can be seen centered on the direct beam as well as the low-index MoS2 spots due to double diffraction. b) SAED of a MoS2 flake with Mo atop it after 4 h at 673 K. The polycrystalline diffraction rings of Mo can be seen and are labeled. Faint double diffraction can be seen around the low-index MoS2 spots. The indexes are labeled, with MoS2 in upright numbers and Mo in italics. et al. upon deposition on mica 35 ). When a large 200 µm selected area aperture is used, discontinuous polycrystalline rings can be seen along with double diffraction around the MoS2 spots. The resulting SAED pattern can be seen in Figure 9 a) and an ADF-STEM image of the sample can be seen in Figure 9 b). Faint double diffraction can be seen. The Re-covered MoS2 is in the bottom left while the Re-covered, holey carbon substrate can be seen in the top right. The polycrystalline nature of the Re film is evident from diffraction contrast, which is unsurprising since this image is from ADF-STEM and not a true HAADF-STEM image. In addition, some similarly sized grains can be seen (although with reduced contrast) on top of the MoS2 flake. The grain size, as deposited, is between 75 and 100 nm. Figure 10 a) and b) a SAED patterns with two different orientational relationships on the same MoS2 flake. Figure 10 a) shows a SAED image of a grain that exhibits an ori-

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Figure 9: a) SAED using a large selected area aperture of a MoS2 flake with Re atop it, as deposited. The discontinuous Re rings indicate a polycrystalline Re film. The Re rings are indexed with italic numbers while the MoS2 spots are indexed with upright numbers. b) An ADF-STEM image of the same sample. HAADF in the corner indicates the detector used. entational relationship of Re(0001)kMoS2 (0001); Re[01¯10]kMoS2 [11¯20]. Figure 10 b) shows another grain in the sample that has an orientational relationship of Re(0001)kMoS2 (0001); Re[11¯20]kMoS2 [11¯20]. In both images, primary Re spots are labeled in italics, but there is extensive double diffraction around each MoS2 spot. The SAED patterns in Figure 10 a) and b) both have a relationship where the basal plane of both MoS2 and Re are perpendicular to the zone axis, but that was not universally seen for the patterns that could be indexed. After annealing for 4 h at 673 K, some regions of orientational relationships exist as before, but largely the sample is similarly difficult to index. There is neither significant grain growth nor a more consistent orientational relationship detected across the MoS2 flake. Very slight double diffraction can still be seen. Figure 11 a) shows the SAED pattern and b) the ADF-STEM image for an annealed Re/MoS2 sample. The grain size has increased from 75 to 100 nm on deposition to 125 to 175 after annealing. As a whole, the Re/MoS2 system is not epitaxial. While there are some specific grains that can be indexed with an orientational relationship such as the above examples, the 16


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Figure 10: a) SAED of a Re on MoS2 as deposited. The grain with an orientational relationship of Re(0001)kMoS2 (0001); Re[01¯10]kMoS2 [11¯20]. b) SAED of another grain, with orientational relationship of Re(0001)kMoS2 (0001); Re[11¯20]k MoS2 [11¯20].

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Figure 11: a) SAED using a large selected area aperture of a MoS2 flake with Re atop it after annealing for 4 h at 673 K. The discontinuous Re rings indicate a polycrystalline Re film. The Re rings are indexed with italic numbers while the MoS2 spots are indexed with upright numbers. b) An ADF-STEM image of the same sample showing the many grains. HAADF in the corner indicates the detector used.

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Figure 12: a) SAED of a Ru on MoS2 on deposition. The Ru is polycrystalline and faint double diffraction rings can also be seen around other MoS2 spots. The Ru is indexed in italic and the MoS2 is normal numbers. b) SAED of Ru on MoS2 after 4 h at 673 K. Both Ru and RuO2 are present and polycrystalline. The Ru is indexed in italic in the top half, the RuO2 is indexed in bold italic in the bottom half, and MoS2 is indexed in normal numbers. majority of the Re grains on the MoS2 flake both on deposition and after annealing cannot be indexed. This sort of quasi-order has been previously reported in the In/WSe2 system, so is not unprecedented. 36 Therefore, the Re/MoS2 relationship can be described as somewhat ordered, but it lacks the epitaxy of other films.

Ruthenium Ru is polycrystalline and not epitaxial on MoS2 upon deposition. The SAED can be seen in Figure 12 a). After annealing for 4 h at 673 K, some of the Ru has oxidized to form an oxide (likely RuO2 ), while some unreacted Ru is still present. Both Ru and the ruthenium oxide is polycrystalline.

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a)

b)

1120

1010 1120

1010 0110

0110

1010

0110

0110

1010 1120

1120

3 1/nm

3 1/nm

Figure 13: a) SAED of a 30 nm Zn thin film on MoS2 on deposition. The Zn is a mixture of epitaxial and polycrystalline. b) SAED of the same flake after 4 h at 473 K. Zn is now entirely epitaxial with the relationship Zn(0001)kMoS2 (0001); Zn[11¯20]kMoS2 [11¯20], but a ±6◦ misalignment angle remains after annealing. Zinc Zinc has a very low melting temperature (693 K), and therefore a high homologous temperature on room temperature deposition. Therefore, the atoms should be very mobile at moderate temperatures. On deposition, the Zn film is a mixture of epitaxial with a large misalignment angle and polycrystalline, and the SAED can be seen in Figure 13 a). There is epitaxial ordering around the MoS2 001 spots due to epitaxy with an orientational relationship Zn(0001)kMoS2 (0001); Zn[11¯20]kMoS2 [11¯20]. The epitaxy is misaligned by greater than ±7◦ , but there are also very faint rings for polycrystalline Zn present. Similar to Al, the combination of double diffraction and misalignment of the Zn atoms on MoS2 results in small hexagons surrounding the MoS2 spots. The same sample was then annealed at 473 K for 4 h and the SAED can be seen in Figure 13 b). The Zn/MoS2 samples were annealed at 473 K instead of the 673 K used in

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the other systems due to the very low melting temperature of Zn (692 K). After annealing, the faint polycrystalline rings seen before annealing disappeared. The entire Zn is epitaxial with the relationship Zn(0001)kMoS2 (0001); Zn[11¯20]kMoS2 [11¯20], but there is still a very large misalignment angle of ±6◦ even after annealing. Again, the combination of double diffraction and a large misalignment angle results in Zn hexagons surrounding the MoS2 spots.

Discussion Typically, for a film to be epitaxial without the introduction of misfit dislocations at the interface, there must be a small lattice mismatch (much less than 1% or the film must be very thin). 14 A small lattice mismatch is not required for a metal to be epitaxial on MoS2 , as can be seen in Table 1. The lattice mismatch values for epitaxial metals vary from -9.7% to 23.7%, while the values for non-epitaxial metals vary from -74.9% to 26.9%. Van der Waals epitaxial materials are much better able to accommodate lattice mismatch, likely due to the lack of directional bonding between the final layer of the van der Waals solid and the metal. However, the choice of metal must have some effect on vdW epitaxy of MoS2 , or else all metals with a plane that has matching symmetry would be epitaxial and they are not. Prior to this study, all known epitaxial metals on MoS2 were FCC. This study confirmed epitaxy in the Ag/ and Pd/MoS2 systems and discovered epitaxy in the Al/, Zn/, and Cu/MoS2 systems, which are all FCC except Zn, which is HCP. All new epitaxial FCC metals discovered in this study continued to adopt the same M(111)kMoS2 (0001); M[2¯20]kMoS2 [11¯20] epitaxy. HCP epitaxial metals have the relationship M(0001)kMoS2 (0001); M[11¯20]k MoS2 [11¯20]. On the other hand, this study found that the metals Mo, Mn, Re, Ru, and Ni were not epitaxial on MoS2 . Mo and Mn are BCC, Re and Ru are HCP, and Ni is FCC. A factor often important in thin-film growth is the homologous temperature. The ho-

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Table 2: Homologous temperatures of various metals. The bold systems are epitaxial. Metal Tdep /Tmelt Re 0.09 Mo 0.10 Ru 0.11 Pd 0.16 Ni 0.17 Mn 0.20 Cu 0.22 Ag 0.24 Au 0.28 Al 0.32 Zn 0.43 Pb 0.49 mologous temperature is the ratio between the deposition temperature and the melting temperature of a material, in degrees Kelvin. As the homologous temperature increases, the atoms are increasingly mobile and are therefore more able to move into energetically favorable positions. 37 Therefore, it would seem reasonable that a larger homologous temperature for the metal would correlate with the formation of an epitaxial film on MoS2 . To calculate the homologous temperature, the deposition temperature for all of our experiments was 298 K, for Au was 373 K, 21 and for Pb was 293 K. 24 Melting temperatures were from the CRC Handbook. 28 All of the epitaxial films tested in these experiments were already at least partially epitaxial upon room temperature deposition. Even the Zn/MoS2 system was mostly epitaxial on deposition but there was also a faint polycrystalline ring of Zn metal. Similarly, no non-epitaxial metals became epitaxial after annealing. Table 2 shows that a higher homologous temperature roughly correlates with epitaxy. We also compared our experimental results with the density functional theory modeling of single atoms on MoS2 by Saidi 31 and found that, in addition to homologous temperature, a low energy barrier for surface diffusion of the metal on MoS2 (termed the jump barrier or diffusion barrier) to be predictive of epitaxy. The diffusion barrier reported in Table 3 is the minimum of the two reported values in Saidi; they reflect the two different diffusion 21



paths from adjacent η 4 (metal atop Mo) spots, one across the η 1 (metal atop S) spot and the other across the η 6 (metal atop the hollow site at the center of the structure) spot, where the number indicates the coordination of the adatom. 31 Unfortunately, not all systems experimentally investigated here were included in his study. Table 3: Minimum diffusion barrier values from Saidi for all tested M/MoS2 systems. 31 Epitaxial systems are in bold.

Metal Ag Au Al Cu Pd Pt Mn Ni Ru

Minimum Diffusion Barrier eV 0.04 0.07 0.28 0.33 0.43 0.60 0.60 0.85 1.23

All the metals that are known to exhibit room-temperature epitaxy on MoS2 have a low calculated diffusion barrier 38 as seen in Table 3. Given the rearrangement necessary to adopt an epitaxial relationship (as shown by the systems which require annealing to fully adopt an epitaxial relationship), the need for a low diffusion barrier is not surprising. An elevated substrate temperature would provide the adatoms with more energy, which could make it possible for additional metals with proper symmetry to grow epitaxially. The metals tested here that were FCC or HCP but not epitaxial were Re, Ru, and Ni. Among these metals, Ni might require the least increase in temperature because its diffusion barrier is the lowest of the three. Prior research has shown that decreasing the deposition rate gives adatoms more time to diffuse on the surface before being buried by the next layer, which can contribute to the growth of epitaxial films. 19 The effects of deposition rate on epitaxial growth were not studied here but understanding the effects of deposition rate on epitaxial growth of metals 22


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on MoS2 would be an interesting extension of this work.

Conclusion Epitaxial relationships in M/MoS2 systems have been found in this study for the FCC metals Al, Ag, Pd, and Cu, for the HCP metal Zn; but not for Mo (BCC), Mn (BCC), Re (HCP), Ru (HCP), or Ni (FCC). All FCC epitaxial systems displayed the arrangement M(111)kMoS2 (0001); M[2¯20]kMoS2 [11¯20], while HCP epitaxial systems reflected a M(0001)kMoS2 (0001); M[11¯20]kMoS2 [11¯20] relationship. Additionally, presence of a closepacked plane with six-fold symmetry, a low metal–MoS2 diffusion barrier, and a high homologous temperature is a predictor of epitaxial metals on MoS2 . Epitaxy could potentially be a factor in the Schottky barrier height of metal contacts to MoS2 and also could be harnessed as a substrate and growth template to form layered heterostructures with specific arrangements between successive layers. Calculation of the diffusion barrier for other TMD/metal systems would allow for the further validation of this prediction via future experimentation.

Acknowledgement The authors thank the National Science Foundation (DMR 1410334) for their support of this project.

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Graphical TOC Entry Electron diffraction was used to study Al, Ag, Cu, Mn, Mo, Ni, Pd, Ru, Re, and Zn deposited on MoS2 at room temperature. The FCC metals Al and Cu join Ag, Au, Pb, Pd, and Pt as epitaxial with M(111)kMoS2 (0001) and M[2¯ 20]kMoS2 [11¯ 20]. Zinc (HCP) oriented as Zn(0001)kMoS2 (0001) and Zn[11¯ 20]kMoS2 [11¯ 20]). Metals with a closepacked plane with six-fold symmetry, high homologous temperature, and low barrier to surface diffusion on MoS2 tend to grow epitaxially at room or low temperature on MoS2 .

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Room Temperature van der Waals Epitaxy of Metal Thin Films on

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