Shell-Like Nanostructures

Aug 29, 2016 - ... Elena H. Sánchez , Pablo Muñiz , Davide Peddis , Roland Mathieu , Kai Liu , Julian Geshev , Kalliopi N. Trohidou , and Josep Nogu...
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Letter pubs.acs.org/NanoLett

Three-Dimensional Self-Assembly of Core/Shell-Like Nanostructures for High-Performance Nanocomposite Permanent Magnets Hailing Li,† Xiaohong Li,*,†,‡ Defeng Guo,†,‡ Li Lou,† Wei Li,† and Xiangyi Zhang*,† †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China College of Science, Yanshan University, Qinhuangdao 066004, China

‡

S Supporting Information *

ABSTRACT: Core/shell nanostructures are fascinating for many advanced applications including strong permanent magnets, magnetic recording, and biotechnology. They are generally achieved via chemical approaches, but these techniques limit them to nanoparticles. Here, we describe a three-dimensional (3D) self-assembly of core/shell-like nanocomposite magnets, with hard-magnetic Nd2Fe14B core of ∼45 nm and soft-magnetic α-Fe shell of ∼13 nm, through a physical route. The resulting Nd2Fe14B/α-Fe core/shell-like nanostructure allows both large remanent magnetization and high coercivity, leading to a record-high energy product of 25 MGOe which reaches the theoretical limit for isotropic Nd2Fe14B/α-Fe nanocomposite magnets. Our approach is based on a sequential growth of the core and shell nanocrystals in an alloy melt. These results make an important step toward fabricating core/shell-like nanostructure in 3D materials. KEYWORDS: nanostructuring, self-assembly, core/shell nanostructure, nanocomposite magnets

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1). These extreme demands constitute a challenge for the fabrication of high-performance nanocomposite magnets.1,13,14 To implement our idea for fabricating 3D core/shell-like nanostructure in a hard/soft composite magnet, we have devised a method of sequentially growing the hard and soft phases in an alloy melt through controlling both the melting and solidification of alloy (Figure 1a−c). Using this method, we have accomplished the first 3D self-assembly of core/shell-like Nd2Fe14B/α-Fe nanocomposite magnets. The resulting magnet exhibits a recordhigh energy product (25 MGOe) that reaches the theoretical limit for isotropic Nd2Fe14B/α-Fe nanocomposite magnets. Our findings make an important first step toward the fabrication of 3D core/shell-like nanostructure for application in highperformance permanent magnets. We chose Nd9Fe85B6 as a model alloy to fabricate core/shelllike Nd2Fe14B/α-Fe nanocomposite magnets, as it has a high Fe content to yield a high soft-phase fraction.15,16 First, we melted the alloy at a low temperature to obtain a melt with ordered (unmelted) clusters (see Figure 1a), as observed in amorphous alloys and Nd−Fe−B melts.17,18 Next, by controlling the meltsolidification process we attained preferential growth of Nd2Fe14B nanocrystals in the melt from the existing clusters (see Figure 1b). Finally, α-Fe nanocrystals grew between the Nd2Fe14B nanocrystals in the melt. We expected this sequential-

anostructuring of magnetic materials is in high demand for their applications in ultrastrong permanent magnets, extremely high-density magnetic-recording media, and biotechnologies.1−5 The fabrication of high-performance magnetic nanomaterials requires an ability to exquisitely control their nanostructuring in order to produce the nanocrystals with wellcontrolled sizes, morphologies, and distribution. But it is an extremely difficult task, particularly for a two-phase nanomaterial. A core/shell architecture enables a precise control of the size, morphology and distribution (placement) of nanocrystals in the core and shell through tuning their growth process,6−10 thus providing a promising approach to manipulate the nanostructuring of a two-phase material. However, previously achieved core/shell nanostructure via chemical approaches is limited to nanoparticles6 and therefore is not appropriate for making practical bulk materials such as permanent magnets. The fabrication of core/shell nanostructure in a 3D material remains a challenge. To address this issue, we propose a strategy for the 3D selfassembly of core/shell-like nanostructure using a sequential growth of the core and shell nanocrystals. As a model material with which to demonstrate the strengths of our strategy, we chose a nanocomposite magnet consisting of magnetically soft and hard phases,11,12 as it requires a precise control of the soft and hard phases at the nanoscale for achieving a high energy product (which indicates the performance of a magnet):1,2 the soft-phase grains must have a small size (≤10 nm), homogeneous distribution, direct contact with the hard-phase grains, and high volume fractions (up to ∼50% required by theoretical model; ref © 2016 American Chemical Society

Received: June 1, 2016 Revised: August 25, 2016 Published: August 29, 2016 5631

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Figure 1. Fabrication and characterization of core/shell-like Nd2Fe14B/α-Fe nanocomposite magnets. (a-c) Schematic depiction of the 3D self-assembly of the Nd2Fe14B/α-Fe core/shell-like nanostructures: (a) Nd9Fe85B6 melt with unmelted (ordered) clusters is obtained at an appropriate melting temperature; (b) the ordered clusters facilitate the growth of Nd2Fe14B nanocrystals during the melt-solidification process; (c) α-Fe nanocrystals grow between the existing large Nd2Fe14B crystals, yielding a core/shell-like nanostructure. (d) XRD pattern of a melt-spun magnet made by ejecting the Nd9Fe85B6 melt at a melting temperature of Tm = 1350 °C. (e) Bright-field TEM image of the melt-spun magnet discussed in panel (d), which exhibits core/shell-like nanostructures. (f) Statistical size distribution of the core and shell grains in the magnet discussed in panel (e) (see the Method section for details). (g) HRTEM image of the core and shell grains in panel (e). (h) Fast Fourier transformation (FFT) of the HRTEM image of the core grain in panel (g). (i) FFT of the HRTEM image of the shell grain in panel (g). (j) HRTEM image of the Nd2Fe14B/α-Fe interface. The dotted line indicates the hard−soft grain boundary.

showed that the magnet consisted of a high fraction of softmagnetic α-Fe and hard-magnetic Nd2Fe14B phases (see Figure 1d). Analysis of the XRD pattern (see the Method section) yielded a volume fraction and grain size of the α-Fe phase of approximately 28% and 14 nm, respectively. Transmission

grown design to facilitate the 3D self-assembly of core/shell-like Nd2Fe14B/α-Fe nanocomposite magnets (see Figure 1c). To check whether this was the case, we characterized the microstructure of a melt-spun magnet made at a melting temperature of Tm = 1350 °C. X-ray diffraction (XRD) studies 5632

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Figure 2. Characterization of melt-spun magnets made at a higher (Tm = 1400 °C) and lower (Tm = 1325 °C) melting temperatures. (a,b) Bright-field TEM image and the statistical grain size distribution of the magnet made at Tm = 1400 °C. The magnet exhibits a grain-dispersed structure. (c,d) Brightfield TEM image and the statistical grain size distribution of the magnet made at Tm = 1325 °C. The magnet shows a coexistence of core/shell-like nanostructure with some large grains that have a direct contact (indicated with the white circle).

contact with each other, without the existence of intergranular phase (see Figure 1j). These results demonstrate that a core/ shell-like Nd2Fe14B/α-Fe nanocomposite magnet with wellcontrolled soft-phase grain sizes (13−14 nm), high soft-phase fraction (∼28%), and ideal soft-phase distribution (with a direct contact with the hard phase) was achieved using our strategy. To tune the self-assembly of the core/shell-like nanostructure, we studied the effect of melting temperature on its formation. We made the melt-spun magnets at higher (Tm = 1400 °C) and lower (Tm = 1325 °C) melting temperatures. We found that the magnet made at higher Tm = 1400 °C showed a grain-dispersed structure (see Figure 2a), whereas the one made at lower Tm = 1325 °C exhibited a coexistence of core/shell-like nanostructure with some large grains that had a direct contact (see Figure 2c); interestingly, the core/shell-like nanostructure showed a small size for both the shell (∼9 nm) and core (∼40 nm) grains (see Figure 2d) as compared with that in the magnet made at Tm = 1350 °C (Figure 1f). This result suggests that the 3D selfassembly of the core/shell-like nanostructure is tunable with melting temperature. However, even at the optimal melting temperature of Tm = 1350 °C, some of the Nd2Fe14B grains are surrounded by less soft-phase grains. Clearly, to form a core/shell-like nanostructure for all the Nd2Fe14B grains, further studies are required by precisely controlling the melting temperature and improving

electron microscopy (TEM) images show a core/shell-like nanostructure in the magnet (see Figure 1e): the core grain has a statistical average size of ∼45 nm and the shell grain has an average size (boundary spacing) of ∼13 nm (Figure 1f) (see the Method section for details). The resulting structure is not a real core/shell nanostructure as achieved in nanoparticles with chemical approaches,6−9 as the α-Fe grains discontinuously distributed surrounding the Nd2Fe14B grains and do not form an actual shell; we therefore name the structure core/shell-like nanostructure. High-resolution TEM (HRTEM) images show that the core grain has crystal faces with interplanar distances of d = 6.10, 4.40, and 3.57 Å (see Figure 1g), which agrees well with interplanar distances of Nd2Fe14B (002) (6.0997 Å), (200) (4.4011 Å), and (202) (3.5688 Å) (see powder diffraction files (PDF): 39-0473). Also, the interplanar angle (φ ∼ 35.7°) between the faces with d = 4.40 and 3.57 Å (see Figure 1h) is similar to the calculated value (φ = 35.8°) between the Nd2Fe14B (200) and (202) crystal faces. These results strongly suggest that the core grain is the Nd2Fe14B phase. The shell grain exhibits crystal faces with an interplanar distance of d = 2.03 Å, similar to that (2.0268 Å) of the α-Fe (110) crystal face (see PDF: 060696). Given the fact that the shell grain also has a size similar to that of the α-Fe phase as determined in our XRD study, we conclude that the shell grain is the α-Fe phase. HRTEM studies further show that the Nd2Fe14B and α-Fe grains are in direct 5633

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Figure 3. Magnetic properties of core/shell-like Nd2Fe14B/α-Fe nanocomposite magnets made by melt spinning at the melting temperature of Tm = 1350 °C, where the largest energy product was obtained (see below). (a) Magnetic-hysteresis loop of the nanocomposite magnet. (b) Energy products (BHi curve) for the magnet discussed in panel (a). (c) Melting-temperature dependence of magnetic properties. The error bars in this panel were determined by measuring three samples made at each melting temperature. (d) Henkel plot (δm−H curve). (e) Magnetic force microscope image of the magnet. The red circle indicates the perimeter of an exchange-coupled magnetic domain. (f) Applied-field (H) dependence of the corecivity (Hci). The step-type increase in the coercivity with increasing the applied field indicates a domain-wall-pinning mechanism.

limit of ∼25 MGOe.19 The magnets made at lower or higher melting temperature show small (BH)max values (see Figure 3c) as compared with the one made at Tm = 1350 °C. These results, combined with the obtained structure information, establish that it is possible to fabricate a nanocomposite magnet with improved energy products by using a core/shell nanostructure. To understand the achieved extremely high energy product in the core/shell-like nanocomposite magnet, we examined the exchange coupling between the hard and soft phases using a Henkel plot20,21 (see Figure 3d). The positive values of δm (as defined in the Method section) are due to exchange interactions promoting the magnetized state.21 The core/shell-like nanostructured magnet exhibited a large value of δm, 0.52, as compared to previously reported nanocomposite magnets (δm = 0.2−0.4) made from Nd9Fe85B6 and the alloys with similar amounts of rare-earth metal (i.e., soft-phase fractions).22−27 These results indicate that the unique core/shell-like architecture allows a strong exchange coupling between the hard and soft phases at a high soft-phase fraction (∼28%).

temperature uniformity in liquid state, as the melting temperature governs the formation of ordered clusters in the liquid state18 that play a key role in producing the core/shell-like nanostructure (see below). Having produced the core/shell-like nanocomposite magnet, we studied whether it exhibited superior magnetic properties (see Figure 3). We measured the magnetic properties of the meltspun magnet at room temperature (along the ribbon direction) with a vibrating sample magnetometer (VSM). We found that the magnet had a high remanence ratio (Mr/Ms = 0.783, where Mr and Ms are the remanent and saturation magnetizations, respectively), large magnetization (4πMs = 16.1 kG for the applied field H = 21 kOe), and high coercivity (Hci = 6.3 kOe) (see Figure 3a), yielding a large energy product, (BH)max = 25 MGOe (Figure 3b). Similar results were repeatedly achieved in the melt-spun magnets made at the same melting temperature (see Figure S1 and Table S1, Supporting Information). The achieved energy product is greater than previously reported values (around 20 MGOe, see below) for isotropic Nd2Fe14B/αFe-type nanocomposite magnets and reaches their theoretical 5634

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Nano Letters The strong exchange coupling is also reflected in the large size (approximately 1000 nm) of the observed exchange-coupled magnetic domains, as indicated in Figure 3e. This is considerably larger than the sizes observed for those of other Nd2Fe14B/α-Fe nanocomposite magnets (150−300 nm).28−30 We believe that the strong exchange coupling at a high soft-phase fraction contributes significantly to the large remanent magnetization (Br = 12.6 kG) (see Figure 3b). We further studied the coercivity mechanism of the core/shelllike nanocomposite magnet by measuring the applied-field dependence of corecivity. A typical step-type increase in the coercivity was observed as the applied field increased (see Figure 3f), indicating a coercivity mechanism that involves domain-wall pinning. A large pinning strength (Hp = 4 kOe) contributes to the large coercivity (Hci = 6.3 kOe). Given small grain size of both the soft and hard phases, the observed large magnetic domain (approximately 1000 nm) suggests that it consists of a lot of soft and hard grains. The grain interfaces might act as thin planar defects to pin the domain walls,22,28,31 contributing to the pinning mechanism. Moreover, the core/shell-like structure separates the hard-phase grains from each other via soft-phase shell grains, which could increase the coercivity through weakening the interaction between the hard-phase grains.31,32 Numerous studies over the past two decades indicated that with increasing soft-phase fraction, the magnetization of nanocomposite magnets is greatly increased at the expense of their coercivity.31,33 The trade-off between the magnetization and coercivity constitutes a formidable challenge when fabricating high-energy-product nanocomposite magnets.31 Strikingly, the achieved core/shell-like nanostructure allows both large remanent magnetization and high coercivity, overcoming the old trade-off between these two qualities in Nd2Fe14B/α-Fe-type nanocomposite magnets11,12,34−45 (see Figure 4a); the core/shell-like nanocomposite magnet therefore exhibits a record-high energy product of (BH)max ∼ 25 MGOe (see Figure 4b), which reaches the theoretical limit for isotropic Nd2Fe14B/α-Fe nanocomposite magnets.19 To understand the mechanism by which the core/shell-like nanostructure forms during the melt-spinning process, we quenched the alloy melt into an amorphous state (in order to preserve the melt structure) from melting temperatures of Tm = 1350 and 1400 °C, and then studied the resulting amorphous structure using XRD (Figure S2, Supporting Information), where the pair distribution function g(r) was employed.46 The alloy quenched from the lower Tm (1350 °C) exhibited a larger ordering range (rm = 1.43 nm) than that quenched from 1400 °C (rm = 1.25 nm) (Figure 5a,b; see the Methods section for details). This suggests that some ordered (unmelted) clusters are likely to exist in the liquid alloy as a result of the low melting temperature, as demonstrated in previous studies of amorphous alloys and Nd−Fe−B melts.17,18 On the basis of these results for the melt structure, we propose a physical mechanism to explain the formation of the core/shelllike nanostructures in the alloy (Tm = 1350 °C). The produced ordered clusters in the liquid alloy are likely to result in a phase separation (with two liquid phases) in the liquid state; the liquid phase that contains the ordered clusters (which may have a structure similar to that of Nd2Fe14B) might facilitate the preferential growth of the Nd2Fe14B crystals during the subsequent solidification process; then, α-Fe crystals could have grown between the large Nd2Fe14B crystals, yielding the observed core/shell-like nanostructure. At higher Tm (1400 °C), large superheating of the melt might destroy the ordered

Figure 4. Comparison of the magnetic properties of nanocomposite magnets. (a) Representative magnetic properties of isotropic Nd2Fe14B/ α-Fe-type nanocomposite magnets. The shaded green area indicates the trade-off between the corecivity (Hci) and remanence (Br). Data marked in red are from the core/shell-like nanostructured composite magnets studied in the present work. Other data were obtained from refs 11, 12, 34−45: half open upward triangle, ref 11; filled upward triangle, ref 12; filled square, ref 34; upward triangle, ref 35; filled downward triangle, ref 36; filled circle, ref 37; square, ref 38; downward triangle, ref 39; half open circle, ref 40; hexagon, ref 41; half open hexagon, ref 42; circle, ref 43; half open square, ref 44; filled hexagon, ref 45. (b) History of the energy product of isotropic Nd2Fe14B/α-Fe-type nanocomposite magnets. Since the discovery of isotropic nanocomposite magnets in 1988, their energy product, (BH)max, has remained around 20 MGOe. Our studies of the core/shell-like nanostructured composite magnets (data marked in red) have led to a significant increase in (BH)max, up to 25 MGOe, demonstrating the strength of the core/shell nanostructure in enhancing energy product.

clusters,18 and thus, no phase separation occurs within the liquid alloy; as a result, a grain-dispersed structure was achieved (see Figure 2a). Therefore, a phase separation with two liquid phases in the liquid alloy might lead to the self-assembly of the core/ shell-like nanostructures in the solidification process, as demonstrated in previous studies of the immiscible alloys.47−49 At an even lower Tm (1325 °C), a Fe3B phase is present in the melt-spun magnet (see Figure 5c). We suggest that a lot of ordered clusters that exist in the melt at the much lower Tm might make the formation and growth of the Nd2Fe14B and Fe3B crystals easier during the solidification process, and thus lead to a direct contact for the preferentially grown grains in some regions (see Figure 3c), which prevents the formation of the core/shelllike nanostructure there. 5635

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Figure 5. Effect of melting temperature (Tm) on microstructure of melt-spun alloys. (a) Pair-distribution function g(r) of amorphous Nd9Fe85B6 made by quenching the alloy melt from Tm = 1350 and 1400 °C. The g(r) curves were obtained from measured XRD patterns (Figure S2, Supporting Information). (b) Zoomed view of the g(r) curves shown in panel (a). The medium-range ordering range, rm, is indicated in the plot. (c) XRD pattern of the melt-spun magnet made by ejecting the alloy melt at a lower melting temperature of Tm = 1325 °C. In addition to the Nd2Fe14B and α-Fe phases, Fe3B was detected in the magnet.

The resulting magnets achieve both large remanent magnetization and high coercivity, thus leading to a record-high energy product of 25 MGOe which reaches the theoretical limit for isotropic Nd2Fe14B/α-Fe nanocomposite magnets. By aligning the hard-phase grains along their easy magnetization axis, an even higher energy product is anticipated in anisotropic core/shelllike nanocomposite magnets. Our findings open up the door to making core/shell-like nanostructure in 3D materials and will spur the research of core/shell nanostructure for advanced applications including ultrastrong permanent magnets. Methods. Materials and Sample Preparation. The Nd9Fe85B6 raw material was produced by arc-melting commercial pure Nd, Fe, and Fe−B (∼21 wt % B) metals. We then fabricated core/shell-like Nd2Fe14B/α-Fe nanocomposite ribbons using a well-controlled melt-spinning technique. First, we melted Nd9Fe85B6 in an Ar environment at an experimentally optimized melting temperature of Tm = 1350 °C. Subsequently, we controlled the solidification process of the melt by melt spinning at a wheel speed of ∼17 m/s. The melting temperature was measured using an infrared two-color quotient pyrometer with lens and light guide technology. The fabricated melt-spun magnets (ribbons) had a thickness of ∼25 μm and a width of ∼2 mm, thus being regarded as a 3D material in this study. To study the effect of the melting temperature Tm on the formation of core/shell-like nanostructure and magnetic properties, we also produced the melt-spun magnets at Tm = 1325, 1375, and 1400 °C. In order to examine the melt structure at Tm = 1350 and 1400 °C, we quenched the melt into an amorphous state at a wheel speed of ∼35 m/s.

Our approach is, in essence, different from conventional rapid solidification (or simple melt-spinning method). According to the Nd−Fe−B phase diagram,50 no phase separation or phase segregation exists for the composition used; and the α-Fe phase is the primary crystal. However, with our method, the ordered clusters were deliberately introduced into the liquid alloy by precisely controlling the melting temperature; these clusters are likely to lead to a phase separation in the liquid state, facilitating the preferential growth of Nd2Fe14B crystals and the selfassembly of the core/shell-like Nd2Fe14B/α-Fe nanostructures during the subsequent solidification process. Our approach should be general and applicable to other alloy systems that possess a phase separation (with two liquid phases) in a liquid state. Such a phase separation could be realized either by composition designs (such as immiscible alloys)48 or by external stimuli as demonstrated here through deliberately introducing ordered clusters into liquid alloys. Moreover, the formation of a core/shell structure also requires a proper volume fraction between the shell and core phases;48 in the present study, a volume fraction of approximately 28% for α-Fe soft phase is achieved in the core/shell-like Nd2Fe14B/α-Fe magnets, and this value can be further tuned by changing alloy compositions. A large-sized bulk magnet can be made for practical applications by grinding the nanocomposite ribbons into powders and then compressing them into a high-density bulk material or by exploring new methods and composition designs that allow a direct fabrication of bulk materials with the desired core/shell nanostructures. In conclusion, we have demonstrated the first 3D self-assembly of the core/shell-like Nd2Fe14B/α-Fe nanocomposite magnets. 5636

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Characterization. The microstructure of the magnets was determined using a TEM (JEOL JEM-2010; 200 kV) and an XRD (Rigaku D/Max-2500/PC; Cu Kα) in a step-scanning mode. For XRD experiments, we measured three pieces of meltspun ribbons (∼25 μm in thickness, ∼ 2 mm in width, and 10 mm in length) for different melting temperatures. To determine the size distribution of the core and shell grains, we analyzed the TEM images obtained from sample regions of ∼25 μm2 using digital micrograph-analysis software (Gatan, Inc., U.S.A.); three samples for each condition were examined. We also calculated the grain size and volume fraction of the soft- and hard-magnetic phases from the measured XRD spectrum using the Rietveld refinement procedure with HighScore Plus software (PANalytical B.V., Inc., The Netherlands). For these analyses, a goodness of fitting σ < 1.5 and a weighted R profile Rwp < 3 were obtained. The structure of the amorphous alloys was studied using XRD. The pair distribution function (PDF), g(r), was obtained from the measured XRD patterns using analysis software package. The short-to-medium range order is manifested by the peaks at small r in the PDF (ref 46): the first peak in the PDF associates with the short-range order, whereas the structural features beyond the first peak to a distance up to 1−2 nm are called the medium-range order; at larger r, the PDF gradually converges to unity, indicating that no long-range order exists. In the present study, the medium-range ordering range rm was determined with the conventional method51 using the threshold |g(r) − 1| ≤ 0.02, beyond which (rm) the peaks in the PDF fluctuate by no more than ±0.02 around the unity. The magnetic domains of the nanocomposite magnets were imaged using a magnetic force microscope (MFM) (Bruker MultiMode 8). Magnetic Properties. The magnetic properties of the meltspun magnets (along ribbon direction) were measured at ambient temperature using a VSM (Lakeshore 7407); three samples were measured for each experimental condition. To study the exchange coupling between the hard and soft phases in the magnets, we made a Henkel plot (δm−H curve),20,21 where δm(H) = [Md(H) − Mr + 2Mr(H)]/Mr. Md(H) is the demagnetization remanence, which was obtained after saturation in one direction and the subsequent application and removal of a field H in the reverse direction; Mr(H) is the magnetization remanence, which was achieved after the application and subsequent removal of a field H; and Mr is the saturation remanence that was used to normalize the δm(H) value. To obtain the applied-field dependence of the coercivity, we measured the minor hysteresis loops at various applied fields.

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Nos. 51471144, 51371074, and 51471145). We thank X. C. Jia and Y. Y. Feng for plotting some of the figures.

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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.6b02210. Magnetic hysteresis loop; XRD patterns; remanent magnetization, coercivity, saturation magnetization, and maximum energy product data. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 5637

DOI: 10.1021/acs.nanolett.6b02210 Nano Lett. 2016, 16, 5631−5638

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DOI: 10.1021/acs.nanolett.6b02210 Nano Lett. 2016, 16, 5631−5638