Growing Vertical in the Flatland Joshua A. Robinson* The Department of Materials Science and Engineering, Center for 2D and Layered Materials, and Center for Atomically Thin Multifunctional Coatings (ATOMIC), Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: The world of two-dimensional (2D) heterostructures continues to expand at a rate much greater than anyone could have predicted 10 years ago, but if we are to make the leap from science to technology, many materials challenges must still be overcome. Recent advances, such as those by Liu et al. in this issue of ACS Nano, demonstrate that it is possible to grow rotationally commensurate 2D heterostructures, which could pave the way toward single crystal van der Waals solids. In this Perspective, I provide some insight into a few of the challenges associated with growth of heterostructures, and discuss some of the recent works that help us better understand synthetic realization of 2D heterostructures. ately to measure new properties.6,7 There was no need to wait on the development of growth techniques to understand the fundamental physics of complex layered systems. Such manual stacking turned out to be a critical step forward toward realizing that vdW heterostructures could lead to novel properties; however, the heterostructure quest began with a much simpler goal: to reduce charge puddles in graphene.6 Hexagonal boron nitride (hBN) was shown to improve the transport properties of graphene, and soon after, many researchers focused on hBN substrates and encapsulating layers as the next big thing in graphene electronics.8,9 However, such layer stacking via mechanical exfoliation (even with the potential for “selfcleaning”)10 often results in imperfect interfaces due to the significant levels of contamination from the transfer process.9 This issue is most easily visualized in Figure 1, where the tortuous shape of the top gate contact is due to the need to avoid contamination “bubbles”. While manually stacked layers are becoming ever more sophisticated,11 the direct growth of such heterostructures is also on the rise. Both vertical and lateral heterostructures were explored nearly in parallel with manual stacking,12,13 and like manual stacking, lateral heterostructures began with investigations of alternating layers of graphene and hBN.14 Subsequently, the vertical growth of dissimilar materials became the focus of several groups, with TMD/graphene vdW heterostructures being the most popular.15 Interestingly, even in the very early vdW heterostructure synthesis experiments,12 it was shown that a large fraction of the MoS2 domains grown on graphene substrates adopted the same orientation as the underlying graphene. This was the first experimental evidence that, even though there is a massive (∼20−23%) lattice
hile research in layered materials is >150 years old,1 the “rise of the Flatland” is largely credited to Giem and Novoselov’s seminal papers in 2004 and 2005.2,3 In parallel to the rush for graphene glory, the next chapter in two-dimensional (2D) materials (beyond graphene) was well underway, with the recent focus in transition metal dichalcogenides (TMDs) being initiated by molybdenum disulfide (MoS2).4 Furthermore, predictions of novel electronic, optical, and chemical phenomena in vertically heterogeneous 2D structures5 introduced new excitement that “there was more to 2D than just 2D”, and that stacking 2D layers could be the ultimate goal toward materials on demand. In many ways, “beyond graphene” has turned out to be even more fruitful for scientists than graphene itself, as evidenced by the rapid growth in materials discovery over the past few years.
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In the current issue of ACS Nano, Liu et al. utilize synchrotron X-ray scattering and atomic resolution scanning tunneling microscopy to demonstrate that rotational commensurability in MoS2 and epitaxial graphene is driven by the energetically favorable alignment of their respective lattices that results in nearly strain-free MoS2. The beauty of these novel materials comes from their interlayer van der Waals (vdW) bonding. Such weak interlayer bonds have allowed experts in mechanical exfoliation (exfoliationists) of graphene to begin stacking layers immedi© XXXX American Chemical Society
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Figure 1. Manually stacked van der Waals heterostructure consisting of hBN, graphene, and MoS2. The top contacts (top gates) are shaped in such a way as to avoid the polymer residue (black spots in figure) that is common in this technique. Adapted from ref 9. Copyright 2014 American Chemical Society.
mismatch between graphene and TMDs, one could potentially realize epitaxial vdW heterostructures. In the current issue of ACS Nano, Liu et al.16 utilize synchrotron X-ray scattering and atomic resolution scanning tunneling microscopy (STM) to demonstrate that rotational commensurability in MoS2 and epitaxial graphene is driven by the energetically favorable alignment of their respective lattices that results in nearly strainfree MoS2. While previous works have demonstrated MoS2 growth on chemical vapor deposition (CVD) graphene12,17 and epitaxial graphene18 using similar techniques, this research showed that complete commensurability between the MoS2 and graphene can exist. The trick to achieving commensurability seems to be increasing the total energy of the system through growth at higher temperatures. Another interesting aspect of the work is the demonstration that the MoS2 domains are aligned with a much narrower azimuthal distribution than MoS2 grown on sapphire.19 The ordering of the various combinations of TMDs on graphene arises from the long-range lattice matching (Figure 2) accompanied by the van der Waals gap that alleviates any strain associated with the short-range mismatch.
Figure 2. (a) Schematic of a TMD/graphene heterostructure illustrating the long-range alignment of the TMD with the underlying graphene. (b) Reciprocal space map of the MoS2/EG heterostructure demonstrating alignment of the MoS2 with EG. Adapted from ref 16. Copyright 2015 American Chemical Society.
angles.20,22 This result is consistent with directly grown heterostructures using graphene as the substrate, where most grown TMDs appear to be well oriented to the underlying graphene when the growth technique has been optimized (Figure 2).12,16,17,23 Defects Should Dominate the Conversation. Defects control everything (nucleation, growth, electronic transport, and optical properties) in these novel materials. It is clear that even in natural crystals,24 there can be a large number of defect types and densities that dominate transport. However, unlike mechanically stacked layers, the quality of the first 2D layer can dictate the layer perfection of all layers above it. This effect is evident in Figure 3, where wrinkles (Figure 3a) and other defects (Figure 3b) in an underlying graphene sheet lead to
Unlike mechanically stacked layers, the quality of the first 2D layer can dictate the layer perfection of all layers above it. When It Is Grown, Mother Nature Decides the Twist Angle. When mechanically exfoliating layers and manually stacking, one can presumably achieve a near-infinite number of alignment possibilities between layers. This approach is now trending in the layer transfer community and has led to new terminology such as “twisted bilayers”, where the twist angle controls the transport properties in graphene/hBN,20 graphene/MoS2,21 and MoS2/MoS2 twisted bilayers.21,22 However, directly grown vdW heterostructures exhibit fixed rotation angles between the layers that satisfy epitaxial growth of the top layer on the underlying layer. As a result, while it is important to develop fundamental understanding of the impact of twist angle on transport and optical properties in vdW heterostructures, when it comes to scalable synthesis processes, Mother Nature will ultimately dictate the twist angle. Luckily, it appears that nature is on our side, and that the strongest interlayer coupling occurs when the layers are at small twist
Figure 3. (a) Scanning tunneling micrograph of WSe2 grown on epitaxial graphene, demonstrating where wrinkles (or other defects) in the graphene occur; multilayer growth dominates the growth morphology. (b) Transmission electron micrograph of MoS2 grown on epitaxial graphene (EG) demonstrating the impact of defects in the graphene “substrate” on MoS2 layer formatting. B
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be used in its pristine form. Nothing is perfect though, as noted in Figure 3, and one must still take care to ensure minimization of EG-related defects such as wrinkles and SiC step edges. Furthermore, graphene provides a conductive substrate, which can preclude its use in a variety of device architectures.
uncontrolled secondary layer nucleation or highly defective top layers. This is a critical topic in today’s discussions regarding transport in graphene/hBN structures,20,25 and will undoubtedly be an area of rich science in the coming years in directly grown vdW heterostructures.26 In fact, Campbell et al.27 recently pointed out that resonant tunneling in symmetric TMD vdW heterostructures can be far superior to that of graphene, but the presence of defects can result in significant band tail states that diminish or even dominate the resonance peak. This dependence should strengthen the resolve of the materials community to develop methods that provide not only layer stacking control, but also tightly control defects, doping, and alloying within the layers. Without this, it will be difficult to realize the intrinsic properties of 2D heterostructures, or to engineer the properties in a controllable manner.
LOOKING FORWARD The previous discussion has focused on a variety of challenges that must be addressed so that we as a community can ensure that the “Flatland” is more than a series of scotch-tape experiments. While exfoliation research provides critical guidance for our understanding on what may be possible, if we are seriously to move “beyond the bench”, we must demonstrate control over defect formation, layer stacking, and 2D/substrate interactions during synthesis of these layers. The perfect synthesis technique or substrate remains to be identified, but graphene is quite attractive for understanding various properties of vdW heterostructures. Considering graphene’s inertness, atomic flatness, and quality, it may remain a substrate of choice for the materials community for some time, but we must also consider that insulating substrates are critical for most device applications. With that in mind, it would be wise to recall the fundamentals in surface passivation/ functionalization of traditional 3D substrates, and consider improved routes for hBN synthesis.
As Liu et al. demonstrate, graphene appears to be an ideal substrate for subsequent layer growth of 2D materials. The Substrate Is Critical. Another beautiful notion about vdW materials is our ability to put them on “anything”. At the extreme, one can imagine using a banana peel as a substrate for biodegradable electronics, but in reality, do we really want to put these layers on anything? This question becomes even more critical when directly growing 2D materials and vdW heterostructures. To date, the substrate choice has been dominated by silicon dioxide (SiO2); however, this decision does not appear to be based on a rigorous scientific investigation, but rather because that is what the exfoliationists use. Instead, if one looks more closely at the impact of the substrate, it quickly becomes evident that SiO2 (or any amorphous substrate in general) is a poor choice for single crystal growth of 2D layers. Rather, traditional epitaxy tells us that one should consider lattice-matched substrates for such a challenge. Although the synthesis of 2D layers on 3D substrates may not be considered truly epitaxial (substrates may not need to be well lattice matched, see Figure 2), the long-range ordering between the substrate and 2D layer leads to quasiepitaxial growth that produces well-ordered layers on the nonlattice-matched substrate. We get some reprieve with 2D materials thanks to the vdW gap, providing a means to achieve epitaxial-like growth on highly mismatched 3D substrates.19 At a minimum, crystalline substrates will be key to achieving single-crystal 2D layers “beyond graphene”. Furthermore, the surface termination (functionalization) of these layers must be carefully considered because it will dominate the interactions between the substrate and 2D layer, and will ultimately dictate epitaxy19 and doping of monolayers during synthesis.28 As Liu et al.16 demonstrate, graphene appears to be an ideal substrate for subsequent layer growth of 2D materials. It typically has no dangling bonds and, therefore, provides an atomically smooth surface, but the “flavor” of graphene is also critical. More specifically, one must take care to ensure the graphene substrate is clean prior to vdW heterostructure synthesis. In the case of CVD graphene, significant care must be taken to ensure that no polymer residue remains following the transfer process; otherwise, the formation of uniform secondary layers is nearly impossible. This criterion ultimately gives a distinct advantage to epitaxial graphene (EG; graphene grown via silicon sublimation from silicon carbide (SiC)), which can
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS J.A.R. would like to thank R. M. Wallace for fruitful discussions and R. Addou for providing STM images. J.A.R. also acknowledges support from the National Science Foundation, Air Force Research Laboratory, Army Research Office, the Defense Threat Reduction Agency, and the Center for the Low Energy Systems Technology. REFERENCES (1) Brodie, B. C. On the Atomic Weight of Graphite. Philos. Trans. R. Soc. London 1859, 149, 249−259. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (3) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453. (4) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (5) Terrones, H.; López-Urías, F.; Terrones, M. Novel HeteroLayered Materials with Tunable Direct Band Gaps by Sandwiching Different Metal Disulfides and Diselenides. Sci. Rep. 2013, 3, 1549. (6) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722−726. (7) Ponomarenko, L. A.; Geim, A. K.; Zhukov, A. A.; Jalil, R.; Morozov, S. V.; Novoselov, K. S.; Grigorieva, I. V.; Hill, E. H.; Cheianov, V. V.; Fal’ko, V. I.; Watanabe, K.; Taniguchi, T.; Gorbachev, R. V. Tunable Metal−Insulator Transition in Double-Layer Graphene Heterostructures. Nat. Phys. 2011, 7, 958−961. C
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