Letter pubs.acs.org/NanoLett
Cite This: Nano Lett. 2018, 18, 2016−2020
Electron-Beam-Induced Synthesis of Hexagonal 1H‑MoSe2 from Square β‑FeSe Decorated with Mo Adatoms John A. Brehm,*,†,# Junhao Lin,*,‡ Jiadong Zhou,§ Hunter Sims,†,# Zheng Liu,§,∥,⊥ Sokrates T. Pantelides,†,#,γ and Kazu Suenaga‡,∇ †
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, United States National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 5, Tsukuba 305-8565, Japan § Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore ∥ Centre for Micro-/Nano-electronics (NOVITAS), School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore ⊥ CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6, 637553 Singapore # Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States γ Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, United States ∇ Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan ‡
S Supporting Information *
ABSTRACT: Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials but also to manipulate them by forming vacancies, nanofragments, and nanowires or joining nanoislands together. In this work, we show that the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1H-MoSe2 from Mo-decorated 2D β-FeSe and simultaneously image the process on the atomic scale. This is quite remarkable given the different crystal structures of the reactant (square lattice β-FeSe) and the product (hexagonal lattice 1H-MoSe2). The feasibility of the transformation was first explored by theoretical calculations that predicted that the reaction is exothermic. Furthermore, a theoretical reaction path to forming a stable 1H-MoSe2 nucleation kernel within pure β-FeSe was found, demonstrating that the pertinent energy barriers are smaller than the energy supplied by the STEM electron beam. KEYWORDS: MoSe2, FeSe, STEM, DFT, electron beam, synthesis
E
fabricate nanowires in any direction and at any position within a monolayer TMD that the microscope operator desires.6 In yet another example, the EB created anion vacancies and helped fuse a bilayer into a novel monolayer material with different stoichiometry.7 In all of these examples, the beam is used either as a scissor to cut up materials or to alter the local stoichiometry of a base material by knocking atoms out of it. In this paper, we pursue an alternative use of the EB that amounts to chemical synthesis: we employ a 2D metal compound decorated by another metal and induce the synthesis of a new 2D metal compound, not merely alloying or replacing one metal with another. Specifically, we synthesize 1H-MoSe2 from Mo-decorated β-FeSe by using a STEM EB to facilitate chemical reactions. Both the precursor compound and
lectron microscopes (EMs) are powerful material-characterization tools that are capable of determining atomic species and their positions in compounds. EMs accomplish this task by using an electron beam (EB) and then collecting the electrons, which have had their energy reduced by interactions with matter, in various detectors. It is well-known that the EB can damage and alter atomic positions in the specimen being examined. Microscopists have harnessed this EM feature, developing techniques that use the power of the EB to realize desirable atomic rearrangements and fabricate novel structures. For example, the EBs can be directed to form vacancies at specific sites, fragment materials into nanoparticles,1 join nanoislands together,2 and create inverse domains.3 Creative techniques have also demonstrated that EBs can be used to fabricate nanoribbons (Mo5S4 within a monolayer of MoS2)4 and nanowires (MoSe within MoSe2)5 in 2D monolayers of transition metal dichalcogenides (TMDs). In this last case, follow-up research has demonstrated that the EB can be used to © 2018 American Chemical Society
Received: December 28, 2017 Revised: January 31, 2018 Published: February 1, 2018 2016
DOI: 10.1021/acs.nanolett.7b05457 Nano Lett. 2018, 18, 2016−2020
Letter
Nano Letters
MoSe2 + 2Fe, shows that ΔH = −0.85 eV/formula unit of MoSe2 for the reaction, indicating that the system, Modecorated β-FeSe, is not at its thermodynamic ground state and would prefer a state of 1H-MoSe2 and Fe. In the DFT formulation, calculations are performed at 0 K. A second calculation under quasi-harmonic approximation constraints shows that even at an elevated temperature of 730 K, the temperature at which β-FeSe converts to δ-FeSe and α-Fe,19 the reaction remains exothermic, with a calculated change in Gibbs free energy of ΔG = −0.78 eV/formula unit of MoSe2. (See the Supporting Information for details on the calculation methods.) The fact that the reaction is exothermic does not mean that it occurs spontaneously, as significant activation energy barriers might have to be overcome to drive the reaction. To decide if the proposed synthesis reaction is energetically feasible, we endeavored to determine a possible atom-by-atom nucleation process and the corresponding energy barriers to see if they can be overcome using the beam-provided energy. The formation of 1H-MoSe2 inside a β-FeSe lattice is a dynamic process: at once, many Mo atoms are coming into the system, many Fe atoms are leaving, and many Se and Mo atoms are moving into MoSe2 positions. To model the nucleation process, we pick a moment in time when a four-hexagon 1H-MoSe2 fragment is present in a minimal void in a β-FeSe lattice. It is assumed that a DFT constant-shape and constant-volume relaxation of a system with one minimum-volume four-hexagon 1H-MoSe2 carved into a β-FeSe lattice captures the relaxed atomic positions for that instant. The shape (and composition) of this patch maintains stoichiometry for MoSe2 and is also chosen because several researchers have visualized triangular growth of 1H-MoSe220−25 rather than chain growth. Starting with this patch, we reverse-engineer, atom-by-atom, how to arrive at it from a perfect, Mo-decorated FeSe monolayer.
the desired product are 2D crystalline structures, which permits the imaging of both materials and their transformation. Because the goal of the current research is the synthesis of a product dissimilar from the reactant(s) and not simply an alloying of a metal into an existing material, the choice of β-FeSe as the compound precursor is appropriate as it is known that the transition metal of one TMD can be alloyed into another TMD, while Mo (and, in fact, most metals), has not been shown to alloy into β-FeSe.8−17 An additional challenge is the growth of a hexagonal structure with prismatic building blocks (1H-MoSe2) from a square structure with tetrahedral building blocks (β-FeSe); see Figure 1.
Figure 1. (a, c) Top views highlighting the hexagonal shape of 1HMoSe2 and the square shape of β-FeSe. (b, d) Side views highlighting the 1H-MoSe2 prismatic building blocks and the β-FeSe tetrahedral building blocks (images made with VESTA).18
Before proceeding to the experiment, a simple first-principles density functional theory (DFT) calculation was performed to check if the choice of precursor and product components was feasible. The difference in enthalpies between the products and the reactants at the stoichiometric limit, 2β-FeSe + Mo → 1H-
Figure 2. Excerpts of the 41-step pathway from pure β-FeSe (Step 0) (not shown) to a four-hexagon 1H-MoSe2 nucleation kernel inside β-FeSe (step 41). See the Supporting Information for the full pathway. 2017
DOI: 10.1021/acs.nanolett.7b05457 Nano Lett. 2018, 18, 2016−2020
Letter
Nano Letters To perform DFT calculations on the above system, an 8 × 8 β-FeSe supercell with a ∼20 Å vacuum in the z-direction is used. This cell size is required given the periodic nature of the calculation: the influence of a four-hexagon MoSe2 patch on other patches in adjacent 8 × 8 β-FeSe supercells is minimized because each is spaced four unit cells of β-FeSe away from one another. Such a supercell is relaxed to a state in which the magnitudes of forces on all atoms are less than 7 × 10−2 eV/ atom. The energy needed to achieve the relaxed configuration per supercell, ET, is 8 eV. It is calculated as:
Figure 3. Change in energy per step (remove 1 Fe, move 1 Se, or add 1 Mo) as the system progresses from pure β-FeSe to the relaxed fourhexagon MoSe2 configuration inside an 8 × 8 β-FeSe supercell. The symbols 1H through 4H represent when a MoSe2 hexagon has been assembled. The inset shows the change in energy per step for the first three steps undergoing relaxation calculations as opposed to SCF calculations.
E[β‐FeSe 8 × 8 supercell] + E T = E[8(1H ‐MoSe2):(Fe128 − 16Se128 − 16)] + 16μFe − 8μMo
(1)
in which 16 Fe atoms have also been removed: two each for each Mo atom added as required for valence balance under the assumption that all Fe atoms and Mo atoms have strictly 2+ and 4+ oxidation states, respectively. A pathway to the formation of this four-hexagon patch is found by reverse-engineering the relaxed system to its pristine pure β-FeSe state in a series of self-consistent field (SCF) calculation steps. Such SCF steps correspond to optimizing wave functions for fixed positions of the atoms. Each step consists of doing one of the following to the system: removing a Mo atom, moving a Se atom, or adding a Fe atom. Given that a four-hexagon MoSe2 patch consists of 8 Mo atoms and 8 Se pairs (16 Se atoms), with 16 Fe having been removed from the system, the minimum number of steps required to perform this process is 40: 8 Mo added, 16 Fe removed, and 16 Se moved. The pathway found in this work achieved the pure state in 41 steps: an extra step is used to move Fe and Se atoms existing in the remaining β-FeSe lattice back to high-symmetry positions. Thus, there are no superfluous moves. A pictorial of the steps of this pathway proceeding from the pure β-FeSe to the relaxed four-hexagon 1H-MoSe2:β-FeSe structure is given in full in Figure S2. Figure 2 presents a 16-panel excerpt of the process. The energy needed to reach each configuration, EC, is evaluated as EC = E[(1H ‐MoSe2):(β‐FeSe)] + yμFe − xμMo
progression of removing 1, 2, and then 3 Fe atoms from pure βFeSe (step numbers 0 through 3) and relaxing the cell at each step to forces below a magnitude of 5 × 10−2 eV/atom. The resulting change in energy per step becomes a maximum of 1.2 eV, a reduction of 0.5 eV from the SCF-evaluated case. The inset of Figure 3 depicts the results of this calculation. Summing the energies for the steps in a cumulative fashion shows that there is an increase in energy of 10.7 eV as the first MoSe2 hexagon is formed inside pure β-FeSe, and an additional 0.4 eV when the second MoSe2 hexagon forms. The assembly of the third hexagon, though, results in a reduction in energy of 0.5 eV, and the assembly of the fourth lowers the energy by a further 2.5 eV. (A cumulative energy plot is provided in Figure S3.) Indeed, from Figure 3, it is clear that most of the steps in the formation of the first hexagon require energy, while most of the steps in the formation of the third and fourth hexagons release energy to the system. These results would seem to indicate that the kernel of 1H-MoSe2 has stabilized, and the system is set to progress from a nucleation stage to a growth stage. We then attempted to achieve the experimental realization as predicted by these theoretical evidence. Under the guidance of the theory, a layered FeSe flake partially covered by a Mo source was prepared (see the Supporting Information for sample synthesis methods). Figure 4 shows the overall morphology of the sample. Thin regions that consist of only a few layers can be found at the edges. The chemical composition of the flake is determined by the energy dispersive X-ray (EDX) spectrum, indicating a Fe:Se ratio of 0.53:0.47, which confirms the presence of the FeSe compound. Also, a small amount of Mo is detected inside the material and is dispersed uniformly without obvious aggregation. The sample was then placed inside an atomic-resolution STEM with ultrahigh vacuum to investigate the possibility of the reaction proposed by theory. The EB is then controlled to continuously focus on a small, thin region of the sample so that high-energy electrons can induce structural and stoichiometric changes. The sample is heated at 620 K to provide additional thermal energy to facilitate the reaction. At 830 K, the FeSe sample evaporates quickly in the ultrahigh vacuum environment (see Figure S4). This result is consistent with the phase diagram of β-FeSe, according to which the material dissociates at 730 K.19 Figure 5 shows several frames of the MoSe2 nucleating and growing out of the β-FeSe lattice atom-byatom as a function of time. The complete movie showing the continuous process is provided in Movie S1.
(2)
where E[(1H-MoSe2):(β-FeSe)] is the energy of the particles in the remaining structure, y is the number of Fe still removed from the structure, and x is the number of Mo remaining in the structure. The energies correspond to upper limits because they do not include the effects of bond weakening by the beaminduced electron−hole excitations or thermal contributions at finite temperatures. Figure 3 shows the change in energy per step for the process. Figure 3 is informative as no move is greater than 1.75 eV for the removal of a Fe atom, no greater than 0.50 eV for the addition of a Mo atom, and no greater than 1.10 eV for the movement of a Se atom within the lattice. This is significant as these energies are below the maximum energy a 60 kV beam can impart to a Fe atom (2.34 eV), a Mo atom (1.36 eV), and a Se atom (1.65 eV), respectively.26,27 Thus, an active EB in the microscope is capable of imparting enough energy to add (Mo), remove (Fe), or move into position (Se from β-FeSe positions to 1H-MoSe2 positions), the atoms to form the 1HMoSe2. It is at this point that one can have a reasonable expectation that a successful synthesis can be performed on this system. The magnitudes of the moves are exaggerated as well because they are determined from SCF steps. Consider an orderly 2018
DOI: 10.1021/acs.nanolett.7b05457 Nano Lett. 2018, 18, 2016−2020
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Nano Letters
Figure 4. FeSe flake used in the experiment. (a) Low-magnification Zcontrast STEM image of the FeSe flake showing the overall morphology. Thin regions can be found at the edges. (b) Energydispersive X-ray (EDX) spectrum of the flake showing its chemical composition and confirming the presence of Mo. The Fe:Se ratio, as quantified by the K-edges of Fe and Se in the EDX spectrum, is approximately 0.53:0.47. The presence of Na presumably comes from the residues during the growth, and Ge and Ta come from the pole piece of the microscope during the collection routine of the EDX detectors.
Figure 5a displays the first snapshot of the scanned region, showing the intrinsic atomic structure of the square β-FeSe, as confirmed by the fast Fourier transformation (FFT) pattern and electron energy loss spectrum (EELS) (Figure 5b). Mo adatoms are found to be decorated randomly on the FeSe lattice as bright spots in the image due to their heavier atomic number (Figure 5a). These bright spots disappear and reappear frequently during the reaction (as seen in Movie S1), suggesting that most of the Mo atoms are on the surface and highly mobile due to excitation by the EB. As the EB continues to scan the same region, the FeSe lattice rapidly changes to an amorphous cluster (a mixture of Mo, Fe, and Se), as shown in Figure 5c,d. Driven by the EB, the chemical reaction proceeds, and a tiny MoSe2 flake composed of several hexagonal unit cells was captured nucleating from the cluster (Figure 5e). It is evident that the EB amorphizes a small region, which can be interpreted as resulting from the EB triggering a number of adjacent nucleation sites. Due to the limited temporal resolution (∼1s/frame), the scanning EB is not fast enough to capture the step-by-step nucleation process proposed by theory. However, the intermediate stage in which a patch composed of several hexagonal units emerges within the FeSe matrix very much resembles the theoretical nucleation and growth description. As the EB continues to impart energy to move the atoms around, the MoSe2 flake begins to grow epitaxially from the edges to a larger size instead of being damaged, as commonly observed in previous reports4,5 (see Figure 5f−i). This pattern may be due to the small size and clean surface of the freshly created flake, which together minimize contamination due to EB etching. It is notable that the number of Mo atoms needed for the growth of a flake of this size (∼50 atoms) is larger than the number observed in the same region of the initial stage shown in Figure 5a. This is because the actual scanning region is larger than the one shown, and nearby Mo atoms are also excited by the EB to migrate into the reaction region to continue the growth. Se vacancies are occasionally created in the flake during the process, consistent with previous
Figure 5. Snapshots of the nucleation and growth of a hexagonal 1HMoSe2 flake out of the square β-FeSe lattice decorated with dispersed Mo atoms. (a) The intrinsic structure of β-FeSe. The fast Fourier transformed (FFT) pattern is shown on the right, showing a perfect square lattice. Bright spots in the image are Mo adatoms, as highlighted by the white arrows. (b) EELS of the FeSe flake. The Se M4,5 edge and Fe L2,3 edge are highlighted. (c−h) Sequential Zcontrast STEM images showing the nucleation and growth process of 1H-MoSe2. The MoSe2 flake was first observed nucleating by attaching to the FeSe lattice then continuously self-assembling into a larger size. (i) Final stage of the hexagonal 1H-MoSe2 flake. The FFT is shown on the right. (j) EELS of the as-synthesized product. The Mo N2,3 edge appears while the Fe edge disappears after the nucleation process, confirming the successful synthesis of the MoSe2 monolayer flake out of the FeSe lattice. Scale bar: 0.5 nm.
reports,6,28 but they are quickly filled up by the surrounding excess Se atoms. The growth stops when the peripheral Mo or Se source is depleted. The final MoSe2 flake shows a perfect hexagonal lattice as confirmed by the FFT pattern (Figure 5i). EELS shows the chemical identity of Mo and Se in the flake, with no obvious Fe left (Figure 5j). A direct comparison of the chemical identity between the precursor and the as-synthesized product is shown in Figure S5. In conclusion, it has been shown that a focused EB in a STEM, operated at 60 kV and at a temperature of 620 K, is capable of manipulating the atoms of a Mo-decorated β-FeSe system to fabricate 1H-MoSe 2 . Simple DFT enthalpy calculations at 0 K and total energy calculations at elevated temperatures indicated the appropriate choice of reactants for this experiment. The probability of successful EB synthesis of other reactant-product combinations can be assessed in the same manner prior to committing microscope and materials resources. More-complicated theoretical work involving the definition of a reaction pathway to form a stable nucleation kernel within a pure substance can be used to assess whether an EB possesses the required energy to move, remove, or add 2019
DOI: 10.1021/acs.nanolett.7b05457 Nano Lett. 2018, 18, 2016−2020
Letter
Nano Letters
Caudel, D.; Burger, A.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Pennycook, S. J.; Pantelides, S. T. Nat. Nanotechnol. 2014, 9, 436−442. (6) Lin, J.; Zhang, Y.; Zhou, W.; Pantelides, S. T. ACS Nano 2016, 10, 2782−2790. (7) Lin, J.; Zuluaga, S.; Yu, P.; Liu, Z.; Pantelides, S. T.; Suenaga, K. Phys. Rev. Lett. 2017, 119, 016101. (8) Li, W.; Hu, L.; Wang, M.; Tang, H.; Li, C.; Liang, J.; Jin, Y.; Li, D. Cryst. Res. Technol. 2012, 47, 876−881. (9) Zhang, X. H.; Tang, H.; Li, C. S.; Chen, S. Chalcogenide Lett. 2013, 10, 403−409. (10) Tedstone, A. A.; Lewis, D. J.; O’Brien, P. Chem. Mater. 2016, 28, 1965−1974. (11) Chua, X. J.; Luxa, J.; Eng, A. Y. S.; Tan, S. M.; Sofer, Z.; Pumera, M. ACS Catal. 2016, 6, 5724−34. (12) Li, X.; Lin, M.-W.; Basile, L.; Hus, S. M.; Puretzky, A. A.; Lee, J.; Kuo, Y.-C.; Chang, L.-Y.; Wang, K.; Idrobo, J.-C.; Li, A.-P.; Chen, C.H.; Rouleau, C. M.; Geohegan, D. B.; Xiao, K. Adv. Mater. 2016, 28, 8240−8247. (13) Schoop, M.; Medvedev, S. A.; Ksenofontov, V.; Williams, A.; Palasyuk, T.; Troyan, I. A.; Schmitt, J.; Casper, F.; Wang, C.; Eremets, M.; Cava, R. J.; Felser, C. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 174505. (14) Shipra, R.; Takeya, H.; Hirata, K.; Sundaresan, A. Phys. C 2010, 470, 528−532. (15) Zhang, Z. T.; Yang, Z. R.; Li, L.; Ling, L. S.; Zhang, C. J.; Pi, L.; Zhang, Y. H. J. Phys.: Condens. Matter 2013, 25, 035702. (16) Nazarova, E.; Balchev, N.; Nenkov, K.; Buchkov, K.; Kovacheva, D.; Zahariev, A.; Fuchs, G. Supercond. Sci. Technol. 2015, 28, 025013. (17) Song, C.-L.; Zhang, H.-M.; Zhong, Y.; Hu, X.-P.; Ji, S.-H.; Wang, L.; He, K.; Ma, X. C.; Xue, Q.-K. Phys. Rev. Lett. 2016, 116, 157001. (18) Momma, K.; Izumi, F. VESTA graphics software package: Visualization for Electronic and Structural Analysis. J. Appl. Crystallogr. 2011, 44, 1272. (19) Okamoto, H. J. Phase Equilib. 1991, 12, 383−389. (20) Gong, Y.; Ye, G.; Lei, S.; Shi, G.; He, Y.; Lin, J.; Zhang, X.; Vajtai, R.; Pantelides, S. T.; Zhou, W.; Li, B.; Ajayan, P. M. Adv. Funct. Mater. 2016, 26, 2009−2015. (21) Ling, X.; Lee, Y.-H.; Lin, Y.; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2014, 14, 464−472. (22) Cain, J. D.; Shi, F.; Wu, J.; Dravid, V. P. ACS Nano 2016, 10, 5440−5445. (23) Jiao, L.; Liu, H. J.; Yi, Y.; Chen, W. G.; Wang, J. N.; Dai, X. Q.; Wang, N.; Ho, W. K.; Xie, M. H.; Cai, Y. New J. Phys. 2015, 17, 053023. (24) Artyukhov, V. I.; Hao, Y.; Ruoff, R. S.; Yakobson, B. I. Phys. Rev. Lett. 2015, 114, 115502. (25) Artyukhov, V. I.; Hu, Z.; Zhang, Z.; Yakobson, B. I. Nano Lett. 2016, 16, 3696−3702. (26) Lee, J.; Zhou, W.; Pennycook, S. J.; Idrobo, J.- C.; Pantelides, S. T. Nat. Commun. 2013, 4, 1650. (27) Reimer, L.; Kohl, H. Transmission Electron Microscopy: Physics of Image Formation; Springer: New York, NY, 2008. (28) Lehtinen, O.; Komsa, H.; Pulkin, A.; Whitwick, M. B.; Chen, M.; Lehnert, T.; Mohn, M. J.; Yazyev, O. V.; Kis, A.; Kaiser, U.; Krasheninnikov, A. V. ACS Nano 2015, 9, 3274−3283.
atoms to change the reactants into the desired products. In this work, calculations indicate that the energies required to remove Fe atoms out of, and add Mo atoms into, pure β-FeSe, and the movement of Se atoms from β-FeSe positions to 1H-MoSe2 positions are smaller than the energies provided by a 60 kV electron beam.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b05457. Additional information on the experimental method including sample preparation and STEM characterization, theoretical methods including vibrational energy calculation, the full theoretical nucleation pathway, and reactant and product characterization. (PDF) A movie of the in situ atom-by-atom reaction. (AVI)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: +1-(516)-5575198. *E-mail:
[email protected]. Phone: +81-(050)-5326-5933. ORCID
John A. Brehm: 0000-0001-9176-7845 Junhao Lin: 0000-0002-2195-2823 Zheng Liu: 0000-0002-8825-7198 Author Contributions
J.A.B. and J.L. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.L. and K.S. acknowledge JST-ACCEL and JSPS KAKENHI (grant nos. JP16H06333 and P16823) for financial support. The theoretical work was supported in part by the U.S. Department of Energy, grant no. DE-FG02-09ER46554, and by the McMinn Endowment at Vanderbilt University (J.A.B., H.S., and S.T.P.). Calculations were performed at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under contract no. DE-AC0205CH11231. Z.L. and J.Z. thank the Singapore National Research Foundation under NRF RF award no. NRF-RF201308, Tier 2 MOE2016-T2-2-153 and MOE2015-T2-2-007, for financial support.
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REFERENCES
(1) Gonzalez-Martinez, I. G.; Bachmatiuk, A.; Bezugly, V.; Kunstmann, J.; Gemming, T.; Liu, Z.; Cuniberti, G.; Ruemmeli, M. H. Nanoscale 2016, 8, 11340−11362. (2) Shin, J. W.; Lee, J. Y.; No, Y. S.; Kim, T. W.; Jin, S.; Choi, W. K. Nanotechnology 2008, 19, 295303. (3) Lin, J.; Pantelides, S. T.; Zhou, W. ACS Nano 2015, 9, 5189− 5197. (4) Liu, X.; Xu, T.; Wu, X.; Zhang, Z.; Yu, J.; Qiu, H.; Hong, J.-H.; Jin, C.-H.; Li, J.-X.; Wang, X.-R.; Sun, L.-T.; Guo, W. Nat. Commun. 2013, 4, 1776. (5) Lin, J.; Cretu, O.; Zhou, W.; Suenaga, K.; Prasai, D.; Bolotin, K. I.; Cuong, N. T.; Otani, M.; Okada, S.; Lupini, A. R.; Idrobo, J.-C.; 2020
DOI: 10.1021/acs.nanolett.7b05457 Nano Lett. 2018, 18, 2016−2020
