Magnetic Plasmon-Enhanced Second-Harmonic Generation onColloidal Gold NanocupsSi-Jing Ding,†,‡ Han Zhang,‡ Da-Jie Yang,§,∥ Yun-Hang Qiu,§ Fan Nan,§ Zhong-Jian Yang,⊥

Jianfang Wang,*,‡ Qu-Quan Wang,*,§ and Hai-Qing Lin*,∥

†School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan 430074, Hubei, China‡Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China§Department of Physics, The Institute for Advanced Studies, Wuhan University, Wuhan 430072, Hubei, China∥Beijing Computational Science Research Center, Beijing 100193, China⊥Hunan Key Laboratory of Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University,Changsha 410083, Hunan, China

*S Supporting Information

ABSTRACT: The magnetic plasmons of three-dimensional nanostruc-tures have unique optical responses and special significance for opticalnanoresonators and nanoantennas. In this study, we have successfullysynthesized colloidal Au and AuAg nanocups with a well-controlledasymmetric geometry, tunable opening sizes, and normalized depths (h/b, where h is depth and b is the height of the templating PbSnanooctahedrons), variable magnetic plasmon resonance, and largelyenhanced second-harmonic generation (SHG). The most-efficient SHGof the bare Au nanocups is experimentally observed when thenormalized depth h/b is adjusted to ∼0.78−0.79. We find that theaverage magnetic field enhancement is maximized at h/b = ∼0.65 and reveal that the maximal SHG can be attributed to thejoint action of the optimized magnetic plasmon resonance and the “lightning-rod effect” of the Au nanocups. Furthermore, wedemonstrate for the first time that the AuAg heteronanocups prepared by overgrowth of Ag on the Au nanocups can synergizethe magnetic and electric plasmon resonances for nonlinear enhancement. By the tailoring of the dual resonances at thefundamental excitation and second-harmonic wavelengths, the far-field SHG intensity of the AuAg nanocups is enhanced 21.8-fold compared to that of the bare Au nanocups. These findings provide a strategy for the design of nonlinear opticalnanoantennas based on magnetic plasmon resonances and can lead to diverse applications ranging from nanophotonics tobiological spectroscopy.KEYWORDS: Asymmetric metal nanostructures, bimetallic nanostructures, gold nanocups, magnetic plasmon resonance,plasmon resonance, second-harmonic generation

Noble-metal nanocrystals exhibit rich plasmonic proper-ties. Their surface plasmon resonances can enhance

many linear and nonlinear optical signals.1−3 The combinationof plasmonics and nonlinear optics gives rise to a new researchfield called nonlinear plasmonics.2,3 Nonlinear opticalprocesses involving multiphoton excitations are much moresensitive to the enhanced local field than linear processes.Various nonlinear optical processes, including second- andthird-harmonic generation,4−15 four-wave mixing,16,17 andmultiphoton luminescence,18−20 have been observed andmeasured on plasmonic metal nanostructures.Second-harmonic generation (SHG) is a second-order

nonlinear optical process, whereby two photons of an incidentlaser at the fundamental frequency (ω0) are absorbedsimultaneously and then one photon at the second-harmonicfrequency (2ω0) is emitted. SHG offers an approach forphoton up-conversion and has applications in photonic devicesand biological spectroscopy.3 SHG is forbidden in centrosym-

metric materials within the electric dipole approximation.2,3

Therefore, great efforts have been made to design and fabricateplasmonic-metal nanostructures with various non-centrosym-metric geometries, such as lithographically fabricated L-,21

T-,22 U-,23 V-,24 and G-shaped nanostructures,10−12 chemicallysynthesized colloidal nanorods,25 triangular nanoprisms,26 andnanocups.27 So far, most studies on SHG enhancements havefocused only on electric plasmon resonances.Many properties and technologies based on electromagnet-

ism require magnetic resonances, such as negative indexmaterials and cloaking.28,29 Magnetic plasmon resonances areusually supported in highly asymmetric nanostructures. Todate, several types of metal nanostructures have beendemonstrated to exhibit magnetic plasmon resonances (for

Received: January 3, 2019Revised: February 1, 2019Published: February 5, 2019

Letter

pubs.acs.org/NanoLettCite This: Nano Lett. 2019, 19, 2005−2011

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example, split rings,23,30 coupled slit-holes,31 fish nets,32

nanosandwiches,33 horseshoes,34 open shells,35−44 and splitballs).45 They have mainly been fabricated by physicalmethods. Only a few types of metal nanostructures have sofar been synthesized by chemical methods to show strongmagnetic plasmons.44 Magnetic plasmon resonances have beensuccessfully used to enhance the SHG of symmetry-brokenmetal nanostructures fabricated by physical methods, whichpermit high excitation powers and give high SHG efficien-cies.46 However, magnetic plasmon resonances in three-dimensional (3D) metal nanostructures fabricated by physicalmethods are hard to optimize and have not been used in dual-resonance antennas for nonlinear enhancement.24,47,48

In this Letter, we have successfully optimized the magneticplasmon resonance of colloidal Au nanocups, created nonlinear

dual-resonance antennas (DRAs) using Au−Ag heteronano-cups for the first time, and demonstrated strikingly enhancedSHG. The colloidal Au and Au−Ag nanocups with controlledasymmetric geometry are synthesized by facile chemicalmethods. The magnetic plasmon resonance and the corre-spondingly enhanced SHG are optimized by adjusting thenormalized depth of the nanocups. The maximal SHG of theAu nanocups is observed and revealed to be caused by the jointaction of the magnetic plasmon resonance and the “lightning-rod effect”. Furthermore, the Au−Ag heteronanocups with Agnanoparticles attached on the edge of the Au nanocups areprepared. They own dual resonances, respectively, at theexcitation and second-harmonic wavelengths and exhibit 21.8-fold enhancement of SHG compared with the bare Aunanocups. This largely enhanced SHG is caused by the

Figure 1. Gold nanocups with different h/b values. (a) Schematics illustrating the growth of the Au nanocups on the PbS nanooctahedrontemplates and the adjustment of the normalized depth (0 < h/b < 1) of the Au nanocups. (b) SEM images of the Au nanocups with different h/bvalues (b = 105 ± 4 nm). The normalized depth h/b and lateral diameter D of each nanocup sample are given above the corresponding SEM image.The Au nanocups have faceted inner surface and relatively rough outer surface, with the largest opening occurring around h/b = 0.50 for thenanocups.

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cooperative magnetic and electric plasmon resonances at thesame hot spot.The colloidal Au nanocups were prepared using PbS

nanooctahedrons as sacrificial templates in three steps,44 asdetailed in the Supporting Information and illustrated in Figure1a. Briefly, the colloidal single-crystalline PbS nanooctahedronswith height b were synthesized at the first step (Figure S1).The following Au overgrowth started preferentially at onevertex of each PbS nanooctahedron to form Au/PbS Janusnanostructures. The hollow Au nanocups were subsequentlyproduced by selectively dissolving the PbS components off ofthe Janus nanostructures. The depth h and the opening size dof the Au nanocup were adjusted to optimize the magneticplasmon resonance by varying the added amount of the PbSnanooctahedrons in the Au growth solution, which has notbeen carefully examined in the previous studies.27,44

Figure 1b presents a set of representative scanning electronmicroscopy (SEM) images of the Au nanocups with h/b = 0.45± 0.03, 0.49 ± 0.03, 0.53 ± 0.03, 0.69 ± 0.03, 0.78 ± 0.03, and0.85 ± 0.04 (b = 105 ± 4 nm). With increasing h/b values, thelateral diameter D along the direction perpendicular to thesymmetry axis of the nanocup increases, but the opening sizedecreases when h/b > 0.5. The inner surface of the synthesizedAu nanocups is faceted,44 but the outer surface is nearlyspherical and relatively rough, with the roughness increasingwith h/b. The similar nanostructures were also observed inanother series of Au nanocups with a larger b = 141 ± 6 nmand h/b = 0.28 ± 0.02, 0.31 ± 0.02, 0.44 ± 0.03, 0.71 ± 0.03,0.79 ± 0.03, and 0.86 ± 0.04 (Figure S2).The prepared Au nanocups exhibit two resonance peaks in

the extinction spectra (Figure 2a,b). The plasmon modes ofthe Au nanocups are highly dependent on the incidence andpolarization directions of excitation light.44 The extinctionspectra of the Au nanocups suspended in aqueous solutionswith random orientations were recorded. The major resonancepeak can be assigned to the magnetic dipole (MD) modeexcited under transverse polarization and the minor peak at thehigh-energy side of the major peak can be partially attributedto the electric quadrupole (EQ) mode.27 The relative intensityof the minor peak prominently increases with h/b. The Aunanocups with different b values show similar dependence ofthe MD resonance wavelength (λMD) on h/b, but the ones witha larger b have higher tunability of λMD in the longer-wavelength region. λMD red-shifts from 707 to 779 nm as h/b isincreased from 0.45 to 0.85 when b = 105 nm (Figure 2a), andit increases from 764 to 873 nm as h/b is increased from 0.28to 0.86 when b = 141 nm (Figure 2b). From the dependencesof λMD on h/b for b = 105 and 141 nm, we can roughlyestimate the λMD value of the prepared Au nanocups with thegiven values of b and h/b.Figure 2c displays the calculated extinction spectra of the Au

nanocups with different h/b values and a fixed lateral diameterD. The calculations were performed using COMSOL Multi-physics (see the Supporting Information for details). Theexcitation light is along the y axis and polarized along the xaxis, where the x and y axes are defined along the two crossededges of the nanocup. The calculation results clearly reveal theMD and EQ resonances (Figure 2d) and reproduce well thekey spectral features observed in the experiments with randompolarization (when h/b ≤ 0.65). The calculated MD resonancewavelength λMD of the Au nanocup with a fixed D = 180 nmincreases with h/b when h/b ≤ 0.65 but slightly decreaseswhen h/b is further increased. In the experiments, the lateral

size D of the synthesized Au nanocups increases with h/b,which results in the monotonous red shift of λMD with h/b.The weak peak around 680 nm in Figure 2c is a mixture of theelectric and magnetic plasmon resonances attributed to theedge of the nanocups (Figure S4).The SHG of the Au nanocups was investigated using a

wavelength-tunable femtosecond laser with a pulse width of∼150 fs (see the Supporting Information for details). Figure3a,b shows the SHG spectra of the Au nanocups with b = 105and 141 nm, respectively. The spectra are all excited at λL =λMD. The SHG intensity is strongly dependent on thefundamental excitation wavelength λL. As shown in Figure3c, all nanocup samples with b = 105 and 141 nm exhibitmaximal SHG intensities when λL = λMD. This indicates thatthe observed SHG signals of the Au nanocups are enhanced bythe MD resonance. As h/b is varied from 0.28 to 0.79, theSHG intensity of the nanocups monotonously increases. Whenh/b is adjusted to 0.79 ± 0.03 for the nanocups with b = 141nm, the intensity reaches the maximum. The very similar h/bdependence of SHG is also observed on the Au nanocups withthe smaller b = 105 nm (Figure 3d), where the SHG intensityreaches the maximum at h/b = 0.78. This suggests that the h/bvalue for the optimized SHG signal is almost independent ofthe b value.

Figure 2. h/b dependence of the plasmon modes of the Au nanocups.(a, b) Measured extinction spectra of the Au nanocups with b = 105 ±4 and 141 ± 6 nm, respectively. λMD monotonously increases with h/bowing to the increased depth h as well as the lateral size D. (c)Calculated extinction spectra of the Au nanocups with h/b = 0.25,0.50, 0.65, 0.70, and 0.75, respectively, with D fixed at 180 nm. λMDincreases with h/b when h/b ≤ 0.65 and then slightly decreases withh/b when h/b > 0.65. (d) Charge distributions of the MD and EQplasmon modes of the Au nanocup with h/b = 0.75. During thecalculations, the excitation is along the y axis and polarized along the xaxis, and the b value of the Au nanocup is 141 nm.

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To reveal the physical origin of the h/b-dependent SHG ofthe Au nanocups, the field distributions of the magnetic fieldH(ω0), electric field E(ω0) and near-field SHG PSHG(2ω0)were calculated at the MD resonance ω0 = ωMD = 2πc/λMD (b= 141 nm, D = 180 nm) (Figures 3e and S3). They are allstrongly dependent on h/b.The average H(ω0) intensity as a function of h/b exhibits

three features (Figure S5): (i) H(ω0) reaches the maximumaround h/b ≈ 0.65; (ii) ∂H/∂(h/b) has a maximum at h/b =0.50; and (iii) the magnetic dipole resonance strengthHMD(ω0) = H(ω0) − H0(ω0) → 0 when the opening size d→ 0. H0(ω0) is the magnetic field induced by the pure electricplasmon resonance at d = 0.The analytic relationship of the magnetic resonance with h/b

for the three-dimensional nanocups is hard to be theoretically

deduced, but we find that the three key features of themagnetic dipole strength can be well described by (see theSupporting Information for details):

H d h b( ) ( / )m mMD 0

2ω ∝ (1)

where 0.5 ≤ m ≤ 3 for the Au nanocups (Figure S5). A groupof functions, xm = dm(h/b)2m with different m values andweight factors, can fit well the entire curve of HMD(ω0) with h/b for both two-dimensional and three-dimensional magneticresonantors.34,45 The empirical formula of the magneticresonance strength is highly valuable for predicting themagnetic extremum parameters of plasmonic open shells,including two-dimensional horseshoes34 and three-dimensionalsplit balls.45

Figure 3. Measured far-field SHG intensities and calculated near-field H(ω0), E(ω0), and PSHG(2ω0) of the Au nanocups. (a, b) Normalized SHGspectra of the Au nanocups with b = 105 and 141 nm and different h/b values, respectively. The Au amount was fixed and the excitation laserwavelength was adjusted to their corresponding λMD values in the measurements. (c) Excitation wavelength dependence of the far-field SHGintensities of the Au nanocups (b = 105 nm, λMD = 760 nm and b = 141 nm, λMD = 850 nm). The SHG reaches the maximum at λL = λMD. (d)Normalized SHG intensities of the Au nanocups with b = 105 and 141 nm and different h/b values. Both samples demonstrate the maximal SHGintensity at h/b ≈ 0.78−0.79. (e) Calculated h/b dependences of the magnetic field H(ω0), electric field E(ω0), and near-field SHG PSHG(2ω0) ofthe Au nanocups on the xz plane (λ0 = λMD). H(ω0) reaches the maximum at h/b ≈ 0.65, and E(ω0) reaches the maximum at h/b ≈ 0.70 owing tothe joint action of the magnetic plasmon resonance and the “lightning-rod effect”. During the calculations, the excitation light is along the y axis andpolarized along the x axis, and the Au nanocups have b = 141 nm and D = 180 nm.

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Compared to the maximal H(ω0) around h/b ≈ 0.65, E(ω0)reaches the maximum at a smaller opening size (a larger h/b,∼0.70), which is caused jointly by the strong magneticresonance in the hollow cavity and the “lightning rod effect”around the edge of the fundamental laser beam. Similarly, the“lightning-rod effect” also affects the emission field of SHG.Figure 3e shows that the strong magnetic field is located in thehollow cavity, and the hot spots of E(ω0, rHS) and PSHG(2ω0,rHS) are approximately located at the same hot spot positionrHS around the edge of the nanocup. The nonlinear hot spot

PSHG(2ω0, rHS) plays a critically important role on the SHGemission.10,11 In addition, the rough outer surface of thenanocups can enhance the local electromagnetic field intensityand the far-field SHG emission owing to the brokensymmetry.49,50

Ag was further overgrown on the Au nanocups to obtainAu−Ag heteronanocups and the synergetic action of themagnetic and electric plasmon resonances on the nonlinearenhancement was investigated (Figure 4a). To clearly revealthe configuration of the overgrown Ag, the Au nanocups with a

Figure 4. Linear and nonlinear optical responses of the Au−Ag heteronanocups. (a) Schematics illustrating the overgrowth of a silver nanoparticleon the opening edge of the Au nanocup. (b) SEM image of the Au−Ag heteronanocups (h/b = 0.52 ± 0.03 for the initial Au nanocups). The insetis a zoomed-in SEM image, showing a silver NP overgrown on a gold nanocup. (c) Extinction spectra of the initial Au nanocups and the resultantAu−Ag heteronanocups. The magnetic and electric plasmon resonances of the Au−Ag heteronanocups match the fundamental and SHGwavelengths of 866 and 433 nm, respectively. (d) Far-field SHG intensities of the Au−Ag heteronanocups and the initial Au nanocups. The SHG ofthe heteronanocups is enhanced by 21.8 times. (e) Electromagnetic field H(ω0), E(ω0), and SHG PSHG(2ω0) contours of a bare Au nanocup, aAu@Ag nanocup (with a continuous Ag shell) and a Au−Ag nanocup (with a Ag NP). (f) Surface charge distribution of a Au−Ag nanocup at thewavelength of 420 nm. For the calculation, the excitation light is along the y axis and polarized along the x axis. The size parameters are h/b = 0.50,b = 170 nm, and D = 195 nm for the bare Au nanocup and Ag shell thickness tAg shell = 6 nm and the Ag NP diameter dAg NP = 29 nm for the Agcomponent.

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large opening (h/b = 0.52 ± 0.03, b = 170 ± 8 nm) wereemployed for Ag overgrowth. The Au−Ag heteronanocupswere produced by carefully adjusting the Ag+ concentration,pH value, and ligand. In general, a high deposition rate ofsilver, which was realized by adding NaOH and supplyingAgNO3 at a high concentration, was found to be favorable forthe preferential overgrowth of Ag nanoparticles on the edge ofthe Au nanocups. Most Au−Ag heteronanocups have a singleAg nanoparticle (NP) on each Au nanocup, and very few oneshave two Ag NPs but with largely different sizes. Figure 4bdisplays a typical SEM image of the Au−Ag heteronanocups.The Ag NPs are attached on the edge of the Au nanocups(Figure 4b, inset). The diameter of the overgrown Ag NPsvaries in the range of 30−50 nm. The elemental mappingimages reveal a thin layer of Ag on the Au nanocup with athickness of approximately 6 nm (Figure S6).The prepared Au−Ag heteronanocups exhibit three plasmon

resonance modes in the wavelength regions of 800−950 nm(MD), 520−590 nm (EQ), and 400−450 nm (EM),respectively, as shown in Figure 4c. EM represents a complexelectric multipole plasmon mode that is caused by the couplingof the small Ag NP on the edge and the Au nanocup, asdiscussed below. After the overgrowth of Ag on the Aunanocups, the resonance width of the MD mode around 870nm is considerably broadened, the peak wavelength λMD isslightly shortened, the EQ peak is blue-shifted from 590 to 550nm, and a new resonance EM peak appears around 433 nm.Via the tuning of the fundamental laser wavelength to 866 nm,and the SHG from the Au−Ag heteronanocups is largelyenhanced compared to that of the initial Au nanocups. Thecorresponding SHG enhancement factor reaches 21.8 (Figure4d).To reveal the physical mechanism of the observed SHG

enhancement of the Au−Ag heteronanocups, we calculated thelocal field distributions of the linear and nonlinear responses oftwo model heterostructures, the Au nanocup with a uniformcontinuous Ag shell (labeled as Au@Ag nanocup) and the Aunanocup carrying a single Ag NP on the edge (labeled as Au−Ag nanocup), as shown in Figure 4e. The MD resonance of theAu nanocup greatly enhances the magnetic and electric fieldaround the Ag NP (h/b = 0.50, b = 170 nm, D = 195 nm,dAg NP = 29 nm), i.e., a large electric field enhancement factor |f(ω0, rHS)| around the Ag NP hot spot is induced by themagnetic plasmon resonance when ω0 = ωMD (Figure 4e).However, the charge distribution on the surfaces of the Ag NPand the Au nanocup at the resonance wavelength of 420 nm isdisplayed in Figure 4f. The interaction between the Ag NP andthe Au nanocup causes a large perturbation on the chargedistributions on the Ag NP and the edge of the Au nanocuparound the junction region. A large electric field enhancement |f(2ω0, rHS)| in these local regions is therefore induced by theelectric plasmon resonance when 2ω0 is in the wavelengthrange of 400−450 nm. As a result, the cooperative magneticand electric plasmon resonance enhancements |f(ω0, rHS)| and |f(2ω0, rHS)| at the same hot spot leads to a very large SHGintensity enhancement factor at rHS:

F f fr r r(2 , ) (2 , ) ( , )SHG 0 HS 0 HS2

0 HS4ω ω ω= | | ·| | (2)

Therefore, extremely strong near-field SHG around the AgNP and the adjacent edge of the Au nanocup is obtained, asshown in Figure 4e. In comparison, the local electric field ofthe Ag shell is much smaller than that of the Ag NP on the Aunanocup at the MD and the corresponding EM resonances.

This indicates that the Ag NP hot spot plays a crucial role onthe largely enhanced SHG of the Au−Ag heteronanocups.In summary, we have synthesized colloidal Au nanocups and

Au−Ag heteronanocups with faceted inner surface andinvestigated their optimized magnetic plasmon resonance andlargely enhanced SHG. For the bare Au nanocups, the maximalmagnetic field enhancement is found to occur at h/b ≈ 0.65,while the most efficient SHG is experimentally observed at h/b≈ 0.78−0.79. The maximal SHG can be attributed to thestrongest electric field around the edge induced jointly by themagnetic plasmon resonance and the “lightning-rod effect”.Moreover, the Au−Ag heteronanocups with overgrown AgNPs synergize magnetic and electric plasmon resonanceenhancements on the same hot spot with double modematching for SHG, which induces 21.8-fold enhancement ofSHG compared with the bare Au nanocups. These findingsprovide a new strategy for the design of nonlinear plasmonicantennas based on magnetic plasmon resonance. Suchnonlinear antennas integrate the highly desired multifunction-alities of multifrequency resonances and high scattering-to-absorption ratios. They will find diverse promising applicationsranging from nanophotonics to biological spectroscopy.However, the measurements of SHG signals at the single-level have remained challenging on our measurement system,but they can provide rich information about the dependencesof the SHG signal on the excitation and emission polarizationdirections and, therefore, opportunities for bettering under-standing the relationship between SHG and different plasmonmodes.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.9b00020.

Synthesis of the colloidal Au nanocups and Au−Agheteronanocups, numerical calculations and theoreticalanalysis of the charge distributions and electromagneticfield enhancement contours of the Au nanocups, TEMand element mapping images of the Au−Ag heteronano-cups (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: jfwang@phy.cuhk.edu.hk.*E-mail: qqwang@whu.edu.cn.*E-mail: haiqing0@csrc.ac.cn.

ORCIDJianfang Wang: 0000-0002-2467-8751Qu-Quan Wang: 0000-0003-0399-0612Author ContributionsS.J.D., H.Z., and D.J.Y contributed equally. S.J.D. prepared thesamples and analyzed the experimental data. H.Z. helped withthe sample preparations and characterization. D.J.Y. wasresponsible for the theoretical modeling and numericalsimulations. Y.H.Q. performed the SHG measurements anddata analysis. F.N. and Z.J.Y. helped with the SHG measure-ments and numerical simulations, respectively. J.F.W., Q.Q.W.,and H.Q.L. were responsible for the project. S.J.D., J.F.W., andQ.Q.W. wrote the manuscript with help from all authors.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the National Key R&D Programof China (grant no. 2017YFA0303402), the National NaturalScience Foundation of China (grant nos. 91750113, 11674254,and 11704416), Hong Kong Research Grants Council (GRF,grant no. 14306817), NSAF (grant no. U1530401), theMinistry of Science and Technology of China (grant no.2017YFA0303404), and the computational resources fromBeijing Computational Science Research Center. The authorsthank Zhenyu Zhang for stimulating discussion.

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Nano Letters Letter

DOI: 10.1021/acs.nanolett.9b00020Nano Lett. 2019, 19, 2005−2011

2011