Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
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Decomposition-Induced Room-Temperature Magnetism of the NaIntercalated Layered Ferromagnet Fe3−xGeTe2 Daniel Weber,† Amanda H. Trout,#,‡ David W. McComb,#,‡ and Joshua E. Goldberger*,† †
Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States Center for Electron Microscopy and Analysis, The Ohio State University, Columbus, Ohio 43210, United States ‡ Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, United States #
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S Supporting Information *
ABSTRACT: The creation of 2D van der Waals materials with ferromagnetism above room temperature is an essential goal toward their practical utilization in spin-based applications. Recent studies suggest that intercalating lithium in exfoliated flakes of the ferromagnet Fe3−xGeTe2 induces a nonzero magnetization at T ∼ 300 K. However, the nanoscale nature of such experiments precludes precise observations of structural and chemical changes upon intercalation. Here, we report the preparation of sodium-intercalated NaFe2.78GeTe2 as well as the investigation into its structure and magnetic properties. Sodium readily intercalates into the van der Waals gap, as revealed by synchrotron X-ray diffraction. Concurrently, the Fe2.78GeTe2 layer becomes heavily charge doped and strained via chemical pressure, yet retains its structure and ferromagnetic transition temperature of ∼140 K. However, we observe the presence of a ferromagnetic amorphous iron germanide impurity over a wide range of synthetic conditions, leading to room-temperature magnetization. This work highlights the importance of strain and electronic control for manipulating the Curie temperature in 2D ferromagnets, while emphasizing the need for careful chemical analysis when exploring phenomena in exfoliated layers. KEYWORDS: Van der Waals magnetism, intercalation, ferromagnetism, 2D materials, Fe3GeTe2, decomposition
T
greater Ni substitutions down to the paramagnetic behavior of metallic Ni 3−x GeTe 2 . These were confirmed via bulk susceptibility measurements. Alternatively, Fe3−xGeTe2 multilayer flakes that were reduced via electrolytic gating with (1− 2) × 1014 cm−2 charge carriers (∼0.25 e− per Fe3−xGeTe2) were reported to have a TC above room temperature. Due to the nanoscale dimensions of the exfoliated flakes, the TC was determined mainly by anomalous Hall measurements. It was proposed that Li intercalates into the vdW gap to accommodate such a large doping density, while preserving the structural arrangement of the Fe3−xGeTe2 lattice. The exfoliated nature of the sample precluded direct structural observation of intercalation. Thus, there remains a strong need to fully elucidate the interplay of intercalation chemistry, structure, and magnetism in Fe3−xGeTe2. Here, we report the discovery, synthesis, structure, and magnetic properties of the layered solid NaFe2.78GeTe2. The phase was synthesized by reductive intercalation of sodium into Fe2.78GeTe2 powder using Na/benzophenone (Ph2CO) (Figure 1a).19 We chose to focus on Na rather than Li, due to
he recent discovery of ferromagnetism in single-layer two-dimensional (2D) van der Waals (vdW) materials1,2 has opened up many exciting new fundamental avenues for scientific exploration.3 This also includes many potential applications that can be realized with single-layer thick materials,3−6 including magnetoresistive tunnel junctions, quantum computing, multiferroics, and spintronics. A major challenge is that, currently, very few exfoliatable magnetic 2D vdW materials exist.2,3,7−15 Among these, Fe3−xGeTe2 (0 < x < 0.3), Fe5−xGeTe2, CrBr3, CrI3, Cr2Si2Te6, and Cr2Ge2Te6 are the only materials discovered thus far to be ferromagnetic in the bulk and stable as exfoliated flakes. From these materials, the iron germanium tellurides have the highest Curie temperatures (TC ). In the well-characterized itinerant ferromagnet Fe3−xGeTe2, the TC is 150 K and increases to 220 K when Fe starts to occupy the vdW gap.9 There is a strong push to discover new materials with TC well above room temperature, which is an essential goal for any practical application. A couple of recent studies explore the possibility of raising the TC of Fe3−xGeTe2 using either chemical substitutions of Fe for Ni16,17 or via electrolyte gating.18 In both cases, increases in the electron count per transition metal give rise to different reported changes in TC. Fe can be partially or completely substituted with Ni, and yet, the TC consistently decreases with © XXXX American Chemical Society
Received: March 28, 2019 Revised: May 29, 2019 Published: June 28, 2019 A
DOI: 10.1021/acs.nanolett.9b01287 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
its ease of detection via X-ray diffraction methods. First, we grew powders of pristine Fe2.78GeTe2 via high-temperature reaction of the elements. We targeted this substoichiometric composition of Fe3−xGeTe2 as prior research indicated that greater Fe content would lead to Fe migration into the vdW gap.9 The vacancy occurs on the Fe2 site, which is the Fe that sits in the center of the layers. The unit cell of Fe2.78GeTe2 can be indexed with the space group P63/mmc and the lattice parameters a = 3.948789(9) Å and c = 16.399318(36) Å at T = 295 K. The narrow full width at half-maximum (fwhm) observed in the X-ray diffraction pattern is indicative of the large average crystalline domain size of >1 μm (Figure 1b). For Na intercalation, Fe2.78GeTe2 was dispersed in dry tetrahydrofuran (THF) in an Ar glovebox and combined with the same molar amount of Ph2CO and 3 equiv of Na. Upon Na addition, the solution turned deep blue, indicating the presence of the Ph2CO anion radical.19 The reaction was sealed and stirred for 22 h at room temperature. Afterward, the supernatant was removed by decantation, and the dark gray powder was washed with THF until the solution remained colorless and was then dried and stored under Ar. After intercalation, the layered morphology and crystallite size with edge lengths of 10−50 μm were retained, as seen in scanning electron microscopy (SEM) (Figure 1c,d). Transmission electron microscopy (TEM) of the NaFe2.78GeTe2 flakes also confirms that the layered morphology is preserved and features a hexagonal electron diffraction pattern with a lattice constant of a ≈ 4.12(5) Å as determined by selected area electron diffraction (SAED, Figure 1e,f). Furthermore, energy dispersive X-ray (EDX) mapping indicates that all constituting elements are contained in the crystalline flakes of NaFe2.78GeTe2 (Figure 1g). Analysis of synchrotron powder Xray diffraction (PXRD) data indicated that the space group P63/mmc was maintained, while the a- and c-lattice parameters increased to a = 4.07238(4) Å (Δa ≈ 0.12 Å, +3%) and c = 18.89906(19) Å (Δc ≈ 2.50 Å, +13%). The increase of the stacking distance by 1.25 Å is typical for the intercalation of unsolvated Na+ ions into the gap of a vdW material, as observed in TiS2/NaTiS2 (Δ ≈ 1.26 Å) and 2H-MoS2/ NaMoS2 (Δ ≈ 1.00 Å).20,21 The Rietveld fit indicated the presence of 8 wt % of unreacted Fe3−xGeTe2 and 5 wt % Na2Te (Figure 2a). Based on an analysis of the peak broadening, the crystalline domain size of the main phase decreased from >1 μm in pristine Fe2.78GeTe2 to 221(3) nm in NaFe2.78GeTe2. The small crystalline domain size coincides with the observation of Na-rich islands at the edges of the layers in TEM-EDX in Figure 1g. For samples prepared with less than 3 equiv of Na, pristine Fe 2.78 GeTe 2 and NaFe2.78GeTe2 coexist as crystalline phases. In contrast to other alkali-intercalated transition metal chalcogenides,22 a continuous change in lattice parameters with increasing alkali ion content, indicative of substoichiometric occupancies, was not observed. Rietveld analysis revealed that the intercalation proceeds in a topotactic fashion. In NaFe2.78GeTe2, the Fe2.78GeTe2 framework is nearly identical, with Na+ ions occupying the octahedral hole in the vdW gap. No improvement in fit can be attained with partial Na occupancy either on the Fe2 site or on the tetrahedral site in the vdW gap. The six Na−Te bond distances are 3.1719(3) Å, which is within the typical range of 3.127 to 3.679 Å for Na in a 6-fold coordination of Te. The Na−Te distance as well as the small increase of the layer stacking distance by 1.25 Å indicate that no organics were
Figure 1. (a) Schematic diagram of Na intercalation into Fe2.78GeTe2. (b) Synchrotron PXRD pattern (λ = 0.412852 Å) of Fe2.78GeTe2 and NaFe2.78GeTe2 with an inset displaying the change of the stacking distance based on the (002) reflection. (c, d) SEM images of Fe2.78GeTe2 and NaFe2.78GeTe2. (e, f, g) TEM image and representative SAED pattern of NaFe2.78GeTe2 as well as EDX maps of the constituting elements. B
DOI: 10.1021/acs.nanolett.9b01287 Nano Lett. XXXX, XXX, XXX−XXX
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scanning calorimetry. The latter showed an onset of the decomposition below 200 °C and two broad peaks of exothermic reactions at 242 and 282 °C, respectively (Figure S4). These transitions were not observed upon cooling, indicating their irreversible nature. The ex situ PXRD patterns in Figure 2b show that the crystal structure of NaFe2.78GeTe2 is already decomposing upon annealing at 100 °C and is completely lost at 200 °C. Instead, reflections corresponding to Na2Te and Fe3−xGeTe2 appear, as well as a very broad reflection that spans from 40.4° to 45.6° 2θ. This peak would encompass either the two most intense reflections expected for nanocrystalline Fe2−xGe, the (012) and (110), or the most intense reflection for nanocrystalline Fe, the (011). Fe3−xGe was excluded as a possible side phase, as the most intense (021) reflection is at 45.9° 2θ. At 100 °C, the broadness of this reflection would correspond to a crystalline domain size of 10 nm. Upon further annealing at 700 °C, minor crystalline reflections corresponding to FeTe2, Fe3Ge, and Fe1.77Ge appear. Stoichiometrically, the decomposition reaction of 2 equiv of NaFe2.78GeTe2 to form Na2Te and Fe3−xGeTe2 would also produce 2.78 Fe, 1 Ge, and 1 Te excess equivalents. Therefore, the presence of Na2Te at room temperature indicates that there must be some amorphous Fe2−xGe phase, as well as excess Fe and Te. The formation of an alkali telluride seems to be a common trend, as indicated by our initial experiments in n-butyl-lithium-treated Fe2.78GeTe2. Annealing at 400 °C also yielded Li2Te and defective Fe3−xGeTe2, pointing to the formation of A2Te (A = alkali) as a thermodynamic sink. To characterize the magnetic behavior of NaFe2.78GeTe2 and Fe2.78GeTe2 powders, we performed SQUID magnetometry as a function of temperature and applied magnetic field (Figure 3). First, Fe2.78GeTe2 shows ferromagnetic behavior with a Curie temperature at 150 K, consistent with previous reports with this stoichiometry, and indicative of the absence of Fe in the vdW gap. The specific magnetic susceptibility χg below the ferromagnetic transition is measured to be ∼2 × 10−2 emu Oe−1 g−1, again consistent with previous reports.9 The M vs H curve features a hysteretic, and thus ferromagnetic, behavior at 5 K. At 200 K, the M vs H is linear, indicative of being in the paramagnetic regime. In the NaFe2.78GeTe2 sample that contains Na2Te and Fe2−xGe, the temperature-dependent magnetic susceptibility shows ferromagnetic behavior with a transition temperature that has a similar onset. However, at low temperatures, the specific magnetic susceptibility increases to ∼4 × 10−2 emu Oe−1 g−1. Furthermore, above the transition temperature, the magnetization is nonzero with a specific susceptibility of ∼2.8 × 10−3 emu Oe−1 g−1 at 300 K. Again, the M vs H curve features hysteretic behavior at 5 K. However, at 200 K, an S-shaped M vs H curve is observed, indicative of magnetic nanoparticles. This S-shaped M vs H curve is still observed at 350 K (Figure S5), our maximum possible measurement temperature. We attribute the sloping magnetization above 150 K to the presence of the observed amorphous Fe2−xGe phase, as well as any undetected Fe. The magnetic susceptibility below 150 K is a superposition of the expected magnetic behavior for the majority NaFe2.78GeTe2 phase as well as the previously mentioned amorphous impurity phases. It is important to point out that the 300 K specific magnetizations of amorphous Fe and
Figure 2. (a) Rietveld refinement of the NaFe2.78GeTe2 diffraction pattern measured using λ = 0.412852 Å with reflections of unreacted Fe2.78GeTe2 (orange) and Na2Te (green). (b) Ex situ PXRD patterns (λ = 1.5406 Å) of NaFe2.78GeTe2 annealed for 12 h at different temperatures as well as Fe2.78GeTe2 for comparison.
cointercalated into the vdW gap. Within the [Fe2.78GeTe2]− layer, the hexagonal arrangement of Fe2/Ge between triangular layers of Fe1 and Te is preserved, but the distances are slightly modified as summarized in the Supporting Information. The larger atomic displacement parameter of the Ge1 site is consistent with its proximity to the neighboring Fe2 vacancy site, coincident with previous reports.23 Detailed crystallographic data is given in Tables S1−S4, and the patterns including the hkl indices are displayed in Figure S1. With 1 equiv of Na+ in the vdW gap, the [Fe2.78GeTe2]− layer carries 6.95 × 10 14 cm −2 additional charge carriers. Furthermore, we were able to identify the presence of an amorphous impurity Fe2−xGe via TEM imaging and EDX, as displayed in Figure S3. The formation of Na2Te and amorphous Fe2−xGe observed by X-ray diffraction and electron microscopy points toward the metastable nature of NaFe2.78GeTe2. Its instability toward decomposition was further characterized by powder X-ray diffraction of annealed NaFe2.78GeTe2 as well as differential C
DOI: 10.1021/acs.nanolett.9b01287 Nano Lett. XXXX, XXX, XXX−XXX
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whole range. The 150 K NaFe2.78GeTe2 transition still exists; however, the χg vs T becomes much more irregular. Furthermore, the 300 K susceptibility is now 2.7 × 10−2 emu Oe−1 g−1, which is 3 orders of magnitude larger than the paramagnetic state of Fe2.78GeTe2, and even larger than the low-temperature ferromagnetic state in Fe2.78GeTe2. Thus, as has been historically the case in previous fields of study with respect to magnetism (i.e., dilute magnetic semiconductors),26,27 trace impurities dominate the magnetic behavior in Na-intercalated Fe2.78GeTe2. It is interesting to note that minimal changes in the TC of the layered phase were observed, despite one full electron being added to Fe2.78GeTe2 upon intercalation. Intercalation not only changes the spin density of this band ferromagnet but also alters the intralayer bond distances and angles, which can dramatically affect the magnetic coupling. For instance, a similar electron density to NaFe2.78GeTe2 can be found somewhere between Fe1.70Ni0.89GeTe2 (∼0.26 additional e− per stoichiometric equivalent, or 1.8 × 1014 additional e− cm−2 ) and Fe 2.13Ni0.41GeTe2 (∼2.1 additional e− per stoichiometric equivalent, or 1.5 × 10 15 additional e − cm−2),17 yet TC drops from 150 K in Fe2.78GeTe2 to 30 K and 49 K in the Ni-substituted materials, respectively. Also, using the first Ni compound as a comparative example, NaFe2.78GeTe2 and Fe1.70Ni0.89GeTe2 are structurally distinct. Along the in-plane direction, the NaFe2.78GeTe2 has a greater tensile strain (a = 4.072 Å) than Fe1.70Ni0.89GeTe2 (a = 3.954 Å), when compared to undoped Fe2.78GeTe2 (a = 3.949 Å). In the cross-plane direction, NaFe2.78GeTe2 is compressively strained by 1% compared to Fe2.78GeTe2 based off of the Te−Te distances that span one layer. A similar 1% compressive strain occurs in Fe1.70Ni0.89GeTe2, when comparing the c-axis. This indicates that strain plays a critical role in tuning the magnetic properties in iron germanium telluride. Additional structural information on the LiClO4 electrolytically gated nanoflakes of Fe3−xGeTe2 (∼0.25 e− per layer unit cell, ∼1.5 × 1014 cm−2)18 with a reported room-temperature ferromagnetism would further confirm if a greater tensile strain along the in-plane direction leads to higher TC values. In conclusion, our experiments show that Fe2.78GeTe2 can be topotactically intercalated with sodium in the presence of benzophenone to yield NaFe2.78GeTe2. Even though one full e− per Fe2.78GeTe2 is added, the TC does not appreciably change. However, we observe evidence of room-temperature magnetism that results from the presence of a Fe2−xGe impurity. This impurity naturally forms under a wide range of conditions on account of the thermodynamic driving force for alkali telluride formation. The magnetic behavior at room temperature is dominated by this impurity phase. This work proves that intercalation can be utilized as a powerful method for electron-doping Fe3−xGeTe2 and its derivatives. However, our observations emphasize the need for careful design of nanodevices and structural characterization of electrolyte-gated magnetic 2D materials.
Figure 3. (a) Temperature dependence of the specific magnetic susceptibility χg of NaFe2.78GeTe2 and Fe2.78GeTe2 at μ0H = 0.01 T. (b) Field-dependent specific magnetization Mg of NaFe2.78GeTe2 and Fe2.78GeTe2.
Fe2−xGe are very similar and range from 80 to 90 emu g−1,24,25 which are much larger than the 2.8 emu g−1 specific magnetization of NaFe2.78GeTe2. Thus, the observed hightemperature magnetization could be explained by the presence of ∼5 wt % of Fe or Fe2−xGe, or any stoichiometric combination of the two. This is very close to the amount Fe and Fe2−xGe that would be expected considering the presence of 5 wt % of Na2Te observed in the synchrotron measurements. To further confirm that NaFe2.78GeTe2 decomposition leads to Fe2−xGe/Fe formation and thus a sloping baseline at T > 150 K, we measured the susceptibility of the 100 °C annealed, further degraded NaFe2.78GeTe2 sample (see Figure 2b). The specific magnetic susceptibility is now much larger over the
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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.9b01287. Additional data and details regarding the synthesis, X-ray diffraction, STEM and SEM measurements, differential D
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(16) Drachuck, G.; Salman, Z.; Masters, M. W.; Taufour, V.; Lamichhane, T. N.; Lin, Q.; Straszheim, W. E.; Bud’Ko, S. L.; Canfield, P. C. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 98 (14), 144434. (17) Stahl, J.; Pomjakushin, V.; Johrendt, D. Z. Naturforsch., B: J. Chem. Sci. 2016, 71 (4), 273−276. (18) Deng, Y.; Yu, Y.; Song, Y.; Zhang, J.; Wang, N. Z.; Sun, Z.; Yi, Y.; Wu, Y. Z.; Wu, S.; Zhu, J.; Wang, J.; Chen, X. H.; Zhang, Y. Nature 2018, 563 (7729), 94. (19) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96 (2), 877− 910. (20) Wang, X.; Shen, X.; Wang, Z.; Yu, R.; Chen, L. ACS Nano 2014, 8 (11), 11394−11400. (21) Haange, R. J.; Bosalberink, A. J. A.; Wiegers, G. A. Ann. Chim. France 1978, 3 (3), 201−207. (22) Dahn, J.; Py, M.; Haering, R. Can. J. Phys. 1982, 60 (3), 307− 313. (23) Nguyen, G. D.; Lee, J.; Berlijn, T.; Zou, Q.; Hus, S. M.; Park, J.; Gai, Z.; Lee, C.; Li, A.-P. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 97 (1), No. 014425. (24) Grinstaff, M. W.; Salamon, M. B.; Suslick, K. S. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48 (1), 269. (25) Kanematsu, K. J. Phys. Soc. Jpn. 1965, 20, 36−43. (26) Lawes, G.; Risbud, A.; Ramirez, A.; Seshadri, R. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71 (4), No. 045201. (27) Shinde, S.; Ogale, S.; Higgins, J.; Zheng, H.; Millis, A.; Kulkarni, V.; Ramesh, R.; Greene, R.; Venkatesan, T. Phys. Rev. Lett. 2004, 92 (16), 166601.
scanning calorimetry, and additional high-temperature magnetic data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Daniel Weber: 0000-0003-4175-9278 Joshua E. Goldberger: 0000-0003-4284-604X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Yaxian Wang for fruitful scientific discussions and gratefully acknowledge Gatan Inc. for the loan of the double tilt vacuum transfer TEM holder. This work was supported by the Center for Emergent Materials, an NSF-funded MRSEC under award no. DMR-1420451. J.E.G. acknowledges the Camille and Henry Dreyfus Foundation for partial support. D.W. gratefully acknowledges the financial support by the German Science Foundation DFG Research Fellowship (WE6480/1). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357.
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REFERENCES
(1) Huang, B.; Clark, G.; Navarro-Moratalla, E.; Klein, D. R.; Cheng, R.; Seyler, K. L.; Zhong, D.; Schmidgall, E.; McGuire, M. A.; Cobden, D. H.; Yao, W.; Xiao, D.; Jarillo-Herrero, P.; Xu, X. Nature 2017, 546 (7657), 270. (2) Gong, C.; Li, L.; Li, Z.; Ji, H.; Stern, A.; Xia, Y.; Cao, T.; Bao, W.; Wang, C.; Wang, Y.; Qiu, Z. Q.; Cava, R. J.; Louie, S. G.; Xia, J.; Zhang, X. Nature 2017, 546, 265−269. (3) Burch, K. S.; Mandrus, D.; Park, J.-G. Nature 2018, 563 (7729), 47. (4) Wang, Z.; Gutiérrez-Lezama, I.; Ubrig, N.; Kroner, M.; Gibertini, M.; Taniguchi, T.; Watanabe, K.; Imamoğlu, A.; Giannini, E.; Morpurgo, A. F. Nat. Commun. 2018, 9 (1), 2516. (5) Jiang, S.; Shan, J.; Mak, K. F. Nat. Mater. 2018, 17, 406−410. (6) Jiang, S.; Li, L.; Wang, Z.; Mak, K. F.; Shan, J. Nat. Nanotechnol. 2018, 13, 549−553. (7) McGuire, M. A.; Dixit, H.; Cooper, V. R.; Sales, B. C. Chem. Mater. 2015, 27 (2), 612−620. (8) Deiseroth, H. J.; Aleksandrov, K.; Reiner, C.; Kienle, L.; Kremer, R. K. Eur. J. Inorg. Chem. 2006, 2006 (8), 1561−1567. (9) May, A. F.; Calder, S.; Cantoni, C.; Cao, H.; McGuire, M. A. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93 (1), No. 014411. (10) Lee, J.-U.; Lee, S.; Ryoo, J. H.; Kang, S.; Kim, T. Y.; Kim, P.; Park, C.-H.; Park, J.-G.; Cheong, H. Nano Lett. 2016, 16 (12), 7433− 7438. (11) Arguilla, M. Q.; Cultrara, N. D.; Baum, Z. J.; Jiang, S.; Ross, R. D.; Goldberger, J. E. Inorg. Chem. Front. 2017, 4 (2), 378−386. (12) Weber, D.; Schoop, L. M.; Duppel, V.; Lippmann, J. M.; Nuss, J.; Lotsch, B. V. Nano Lett. 2016, 16 (6), 3578−3584. (13) Stahl, J.; Shlaen, E.; Johrendt, D. Z. Anorg. Allg. Chem. 2018, 644 (24), 1923−1929. (14) May, A. F.; Ovchinnikov, D.; Zheng, Q.; Hermann, R.; Calder, S.; Huang, B.; Fei, Z.; Liu, Y.; Xu, X.; McGuire, M. A. ACS Nano 2019, 13, 4436−4442. (15) Lin, M.-W.; Zhuang, H. L.; Yan, J.; Ward, T. Z.; Puretzky, A. A.; Rouleau, C. M.; Gai, Z.; Liang, L.; Meunier, V.; Sumpter, B. G.; Ganesh, P.; Kent, P. R. C.; Geohegan, D. B.; Mandrus, D. G.; Xiao, K. J. Mater. Chem. C 2016, 4 (2), 315−322. E
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