Aluminum Film-Over-Nanosphere Substrates for Deep-UV Surface-Enhanced Resonance Raman SpectroscopyBhavya Sharma,‡,† M. Fernanda Cardinal,† Michael B. Ross,† Alyssa B. Zrimsek,† Sergei V. Bykov,§

David Punihaole,§ Sanford A. Asher,§ George C. Schatz,† and Richard P. Van Duyne*,†

†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States‡Department of Chemistry, University of Tennessee, 1420 Circle Drive, Knoxville, Tennessee 37996, United States§Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States

*S Supporting Information

ABSTRACT: We report here the first fabrication ofaluminum film-over nanosphere (AlFON) substrates for UVsurface-enhanced resonance Raman scattering (UVSERRS) atthe deepest UV wavelength used to date (λex = 229 nm). Wecharacterize the AlFONs fabricated with two different supportmicrosphere sizes using localized surface plasmon resonancespectroscopy, electron microscopy, SERRS of adenine, tris-(bipyridine)ruthenium(II), and trans-1,2-bis(4-pyridyl)-ethyl-ene, SERS of 6-mercapto-1-hexanol (as a nonresonantmolecule), and dielectric function analysis. We find thatAlFONs fabricated with the 210 nm microspheres generate anenhancement factor of approximately 104−5, which combinedwith resonance enhancement of the adsorbates provides enhancement factors greater than 106. These experimental results aresupported by theoretical analysis of the dielectric function. Hence our results demonstrate the advantages of using AlFONsubstrates for deep UVSERRS enhancement and contribute to broadening the SERS application range with tunable andaffordable substrates.

KEYWORDS: UV resonance Raman spectroscopy (UVRRS), surface-enhanced Raman spectroscopy (SERS), aluminum plasmonics,film-overnanosphere substrates

Surface-enhanced Raman scattering (SERS) relies on theelectromagnetic (EM) field enhancement by collective

oscillations of conduction electrons in metal nanomaterials,known as localized surface plasmon resonance (LSPR). Sincethe discovery of SERS,1 the types of substrates that supportsurface enhancement have been expanded from roughenedelectrodes to nanoparticles and various solid-support sub-strates.2 These substrates, usually made of coinage metals, suchas Ag, Au, and Cu, are well characterized and enhance Ramanscattering in the visible to infrared wavelength range.2 TheLSPR of Au or Ag nanostructures amplify the inherently weakRaman signal by up to 108.3

One wavelength region of the spectrum that has remainedelusive for SERS enhancement is the deep ultraviolet (DUV).Theoretical calculations predict that metals, such as aluminum(Al), gallium (Ga), indium (In), lead (Pb), tin (Sn), and otherswith negative real and small imaginary dielectric constants inthe UV, are viable materials for SERS.4−6 The most commonmetals used experimentally to attempt UVSERS include Al,7−11

Ga,12 Rh, Ru, Pt, and Pd.13−18 Here, we will focus on the use ofAl as a SERS substrate. Previous reports of UVSERSenhancement on Al have achieved enhancement factors (EFs)of ∼50 using Al nanoparticle array substrates and 257.2 nm

excitation;9 ∼103 on Al nanovoids with adenine excited at 244nm;7 and for tip-enhanced Raman spectroscopy, using a siliconcantilever with Al deposited on it, adenine was probed with 266nm excitation and the calculated EFs were in the 60−200range.19

In addition to harnessing the enhancement due to plasmonexcitation in the UV, the excitation of many molecules in theDUV frequency range (190−300 nm) is highly desirablebecause it results in enhancement due to DUV resonanceRaman scattering (UVRRS). UVRRS becomes prevalent whenthe excitation wavelength falls within an electronic absorptionband, exciting a specific chromophore of the target molecule.This excitation, which is in resonance with an electronictransition of the molecule, results in enhancement of thenormally weak Raman signal by 2 to 6 orders of magnitudedepending on the magnitude of the extinction coefficient of theelectronic transition and its excited state line width. Addition-ally, UV excitation below 250 nm minimizes contaminatingfluorescence backgrounds.20 Combining UVRRS with SERS

Received: October 13, 2016Revised: November 8, 2016Published: November 10, 2016

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(UVSERRS) allows for highly sensitive and selective enhance-ment of adsorbed molecules at ultralow sample concentrations.Herein, we present Raman spectra excited at the deepest UV

wavelength used to date (λex = 229 nm) for four analytes on thefollowing: SERS measurements on Ag and Al film overnanosphere (FON) substrates, and resonance Raman measure-ments on Ag and Al mirrors, quartz, in solution, and in the solidstate. To estimate the contribution of the resonance enhance-ment, we also excited SERS at two different wavelengths (λex =355 and 405 nm) where the molecules show minor absorbance(all Raman instrumentation discussed in detail in SI). Weadditionally employ theoretical arguments to support ourexperimentally observed enhancements. The data presentedhere fully characterizes our Ag and Al SERS substrates andallows us to further understand their performance in the deepUV wavelength region.In our experiments, adenine, tris(bipyridine)ruthenium(II)

(Ru(bpy)32+), 6-mercapto-1-hexanol (MH), were obtained

from Sigma-Aldrich and used as received; whereas, trans-1,2-bis(4-pyridyl)-ethylene (BPE) was recrystallized. We measuredUV−visible absorption spectra for 0.125 mM adenine, 0.01 mMRu(bpy)3

2+, 0.01 mM BPE, and 0.5 mM MH with a Cary 5000UV−vis-NIR spectrophotometer (Agilent). From these meas-urements, we see that exciting these molecules with λex = 229,355, and 405 nm will probe their Raman spectra in preresonantand nonresonant states (Figure 1A). In particular, MH wasincluded in our measurements at 229 nm as a nonresonantadsorbate example.FON SERS substrates were used to enhance the UVSERRS

signal of the adsorbate molecules investigated. The method-ology to create AgFONs is well established,21 although not forthe wavelength range of interest. We adapted the AgFONmethodology for the preparation of Ag and Al FONs used here(see SI for substrate fabrication details). FONs are ideal SERSsubstrates for several reasons, including low cost, ease offabrication, and the ability to tune the LSPR λmax to matchexcitation wavelengths by changing the spheres size or amountof metal deposited. Additionally, we have previously demon-strated that noble metal FONs (Ag and Au) have reproducibleenhancement over large areas21 and are stable at hightemperatures (500 K),22 at negative applied potentials(electrochemical),23,24 on the shelf for more than 9 monthswith a protective alumina layer deposited by atomic layerdeposition,25 in biological fluids,26 and in vivo for more than 17days.27

The optical properties of the FONs were measured using aCary 5000 double spectrophotometer in specular reflectancemode (Varian internal DRA accessory) and are shown in Figure1B,C. Previous work from the Van Duyne group28 onsubstrates fabricated by nanosphere lithography establishedthat the “ideal” wavelength of maximal signal from the LSPRphenomenon (LSPR λmax) is equal to 1/2(λex + λvib), where λexis the excitation wavelength and λvib is the wavelength wherethe vibrational mode of interest occurs. For the adsorbatesbeing investigated here, the “ideal” LSPR λmax should fallbetween 234−238 nm (based on λex = 229 nm and vibrationalmodes in the fingerprint region of ∼700−1650 cm−1). Thisindicates that the AlFON prepared with 170 nm spheres shouldprovide the highest signal enhancement at λex= 229 nm,assuming the ideal LSPR condition.Figure 2 shows the SERS spectra of the three resonant

adsorbates (columns) on both Ag (2A, 2D, 2G) and Al (2B, 2E,2H) FONs and mirrors. We find the SERS spectra for BPE,

Ru(bpy)32+, and adenine are shifted from the corresponding

normal Raman spectra in solution by 1−30 cm−1, which is ingood agreement with the frequency shift criteria for SERS (seekey factors to consider for characterizing SERS substrates inSI). In Figure 2A, we show the SERS spectra of BPE onAgFONs made from the 170 and 210 nm sphere diameters. Wesee little enhancement (<10×) of the Raman spectrum of BPEon the Ag mirror. The spectra are consistent with previouslyestablished SERS spectra of BPE,12,29 with characteristic peaksat 1008, 1200, 1604, and 1640 cm−1. On the AgFON 170 nm,the intensities of the 1604 and 1640 cm−1 doublet of BPE areenhanced approximately 10 times. On the AgFON 210 nm, theenhancement is larger and the doublet becomes very wellresolved, along with other BPE peaks clearly evident (1200 and1008 cm−1). A similar pattern of BPE enhancement is seen onthe AlFONs and mirror (Figure 2B). There is little enhance-ment of the BPE signal on the mirror and AlFON 170 nm, butthe enhancement of the AlFON 210 nm substrate issignificantly larger. Although the doublet is less well resolvedhere, the peaks at 1200 and 1008 cm−1 are clearly visible.Comparison of the scale bars in Figure 2A,B show that theAlFON achieves ∼600 times greater intensity than the AgFONfor the BPE SERS signal.

Figure 1. (A) UV−vis absorption spectra (in arbitrary units) of 0.01mM Ru(bpy)3

2+ (black), 0.01 mM BPE (red), 0.125 mM adenine(purple), and 0.5 mM MH (blue) with the laser excitation wavelengthsindicated by the dashed black lines. LSPR spectra (shown as %reflectance) of FONs fabricated using 170 nm (black), 210 nm (red),and 300 nm (blue) spheres for Al (B) and 280 nm (blue) spheres forAg (C). The wavelengths at which the maximal LSPR signal (λmax)occurs are labeled for each LSPR spectrum.

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The second column of Figure 2 includes similar comparisonbetween Ag and Al for Ru(bpy)3

2+. The SERS spectra forRu(bpy)3

2+ on both metals are comparable to those found inthe literature.30−34 In Figure 2D, we find that the AgFON 170nm has approximately 3−5 times higher enhancement than theAgFON 210 nm and the Ag mirror. The AlFON 210 nm hasapproximately 120 times greater enhancement than the AlFON170 nm and Al mirror (Figure 2E).In the case of adenine, third column of Figure 2, the AgFON

170 nm shows a larger enhancement (Figure 2G) than theAgFON 210 nm. We see signal intensities on AgFON 210 nm,which are 10 times less than on AgFON 170 nm and with littlesignal intensity on the Ag mirror for 1 mM adenine. For Al(Figure 2H), we see once more that the AlFON 210 nmprovides greater signal enhancement over the AlFON 170 nmand the Al mirror. In this case, the enhancement of adenine onAl is ∼160 times greater than on Ag. In comparing the spectra

of adenine on the Ag and Al FONs with previously publishedUVSERRS results,7,9,11 we note that the spectra here aredifferent. This illustrates the importance of exciting at deeperUV wavelengths, because we are able to access higher lyingexcited states than the previous studies, resulting in excitationof different vibrational modes. The differences in the UVresonance Raman spectra of adenine and adenosine fromexciting the molecules at different excitation wavelengths(between 200 and 300 nm) have been noted extensively inthe literature, and we find that our peaks correspond well withthe peak assignments in the literature.35−39 We also note thatthe spectra of the different molecules are not the same on Al incomparison with Ag. We believe that this could be due tosurface selection rules for SERS, which indicate that the Ramanmodes of the molecule that are oriented perpendicular to thesurface will be preferentially enhanced.40,41 Additionally, in thecase of Ru(bpy)3

2+ the spectra on Al highly resemble the

Figure 2. SERS and resonance Raman spectra excited at 229 nm. FONs and mirrors were prepared with two different metals: the first rowcorresponds to silver (A, D, G) and second row to aluminum (B, E, H). Additionally, the FONs were made from different sphere sizes, 170 nm (red)and 210 nm (blue). FONs, quartz slides, and mirrors were incubated in 1 mM BPE (first column, A, B, C), 10 mM Ru(bpy)3

2+ (second column, D,E, F), and 1 mM adenine (third column, G, H, I) and subsequently rinsed. Raman spectra acquired on metal mirrors are illustrated with green tracesand on quartz slides with pink traces (C, F, I). The corresponding solution spectra (100 mM BPE, 100 mM Ru(bpy)3

2+, 5 mM adenine) are alsoincluded with purple traces (C, F, I). Peaks labeled with O2 represent the position of the atmospheric oxygen peak (∼1555 cm−1). All peakassignments in black correspond to the literature values of the adsorbates. In E, the peak assignments in red correspond to peaks associated with 2,2′-bipyridine as opposed to Ru(bpy)3

2+. From direct comparison of SERS spectra collected with Ag and Al FONs, we observe that AlFON 210 nmprovides the highest signal-to-noise ratio (S/N).

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spectra of 2,2′-bipyridine (Figure 2E), which may be due to anorientation of the adsorbed molecule on the surface thatenhances those Raman modes. In addition, the electronictransition excited at 229 nm may be localized on the 2,2′-BPYligand, thus enhancing these vibrations and containing littlemetal-to-ligand charge transfer (MLCT) character. For thenonresonant molecule (MH), we were only able to recordRaman spectra of the high wavenumber region, because there islittle resonance enhancement of the Raman signal (Figure S2).Up to this point we have been referring to enhancements in

terms of increased signal intensity, which provides a generalpicture of the overall enhancements but does not account forfactors such as surface roughness (for FON vs mirror), numberof molecules contributing to signal, and so forth. To accountfor these factors, fully characterize our substrates, and to gain adeeper understanding of the mechanisms influencing the signalenhancement, we calculated the SERS and resonance RamanEFs. We define the enhancement on the FONs as a total EFwith contributions from both resonance Raman (RR) and theEM enhancement mechanism of SERS (EFtotal = EFSERS ×EFRR; Figure 3). We attribute the enhancement on the mirrors

to resonance Raman enhancement. When comparing theenhancement of Ag and Al mirrors for Ru(bpy)3

2+, BPE, andadenine, we find a similar order of magnitude of enhancementfor each adsorbate (Figure 3). For example, the EFRR foradenine is ∼2 × 103 on Ag mirror and ∼1 × 103 on Al mirror.Achieving similar EFs on the two mirrors supports ourassertion that the Raman signals on the mirrors arise from aresonance effect. Additionally, because MH undergoes littleresonance enhancement we attribute its entire signal to the EMenhancement effect. Our results show that for MH on AlFON210 nm the EFSERS is ∼2 × 105 and on AgFON 210 nm theEFSERS is ∼4 × 102. Because of the lack of resonanceenhancement, we were unable to collect a signal for MH oneither Al or Ag FONs 170 nm or the mirrors.

Experimentally, the AlFON 210 nm proved to be the mosthighly enhancing substrate, generating total EFs in the 105−107range, whereas the other FONs generated total EFs of 102−104(Figure 3). This is in disagreement with our hypothesis that theAlFON 170 nm would be the most highly enhancing substratebased on the “ideal” LSPR λmax equation determined byMcFarland et al. (2005).28 However, this disparity may beexplained by the fact that the “ideal” LSPR equation wasdeveloped for periodic particle arrays (PPA), which are morehighly ordered substrates than FONs. The less well-orderedstructure of the FONs is due to different shapes andnanostructures on the FON surface which means the nearfields the molecules are experiencing are probably very differentthan the “ensemble” LSPR, or far fields, collected via UV−visabsorption spectroscopy. Consequently, we report here the firstfabrication of FONs with Al, demonstrate that the behavior ofFONs fabricated with different metals cannot be generalized,and we establish that we cannot determine optimal enhance-ment based solely on sphere size and LSPR λmax.In Table 1, we outline the total enhancements for each

molecule on AlFONs 210 nm, as well as the enhancements due

to the EM mechanism (EFSERS) and resonance Raman (EFRR).We find that the EM mechanism contributes more to theenhancement of BPE, Ru(bpy)3

2+, and MH (104−105) thanadenine (103). In adenine, the EM mechanism and theresonance mechanism look to be contributing equally to thesignal. The resonance Raman enhancement is found 103 foradenine and slightly lower for Ru(bpy)3

2+(101) and BPE (102).To probe further into the behavior of the Ag and Al FONs,

we use the optical constants from Johnson and Christy42 for Agand from Palik43 for Al in an analysis of the dielectric functionsto explain the observed enhancements (Figure 4). Notably, anLSPR can only be supported when the real part of the dielectricfunction is negative (Figure 4A, bottom), suggesting that Al willbe “plasmonic” in the UV while Ag will not be. Additionally, weuse the dielectric functions to calculate a plasmon quality factor(QSPP = εR

2/εi),44 which describes the field confinement of the

surface plasmon polariton and which is optimized at eachwavelength with respect to particle shape (Figure 4B).Comparisons with experiment show that the relationshipbetween EF and QSPP varies as EF = E4 = α·QSPP

2, where αis a geometry-dependent prefactor. We have previouslydetermined the α-factor to be 10.5 for Ag immobilized nanorodarrays (AgINRA) substrates, which are similar to FONs exceptthat in the INRA nanorod-type structures are developed on thesurface of the deposited sphere mask.21 The original workincluded experimental results collected with laser excitations at633, 785, and 1064 nm. We have added two additional datapoints at 488 and 532 nm with the original QSPP

2 determination(Figure 4B).

Figure 3. Ensemble averaged enhancement factors at 229 nm for thefour adsorbates investigated, Ru(bpy)3

2+, BPE, Adenine, and MH, onAg and Al FONs and mirrors. The mirrors show the resonance Ramanenhancement. AlFON 210 nm results in the largest total enhancement∼3 × 107 for BPE and SERS enhancement ∼2 × 105 for MH. Theerror bars reflect standard deviation in signal intensity for multiplepeaks on each substrate for 6−12 spectra per substrate.

Table 1. Ensemble Averaged Enhancement Factors for EachMolecule on the AlFON 210 nm Broken down by Total EF,EF Due to Resonance Raman Effect (EFRR) and EF Due toElectromagnetic Enhancement Mechanism of SERS (EFSERS)

total EF EFRR EFSERS

Ru(bpy)32+ 1.96 × 106 3.26 x101 6.01 × 104

BPE 2.76 × 107 1.38 × 102 2.00 × 105

adenine 1.55 × 106 1.44 × 103 1.08 × 103

MH 2.02 × 105 1 2.02 × 105

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In Figure 4C,D (and Figure S3), we plot the QSPP2 results for

pure Ag, pure Al, and an approximate result for Al coated withAl2O3. Previous results suggest that the optimal enhancementby Al with a native Al2O3 coating (Al-Ox) is ∼167 times lessthan that for Al, so we implement 167 as a first approximationto 1/α.5 Figure 4C shows that the EFs measured forRu(bpy)3

2+ on AlFONs 170 nd 210 nm at λex = 229 nm fallbetween the Al and Al-Ox quality factors, indicating consistencywith this crude EM enhancement factor at lower wavelengths(see Figure S3 for Adenine and BPE).We also measured experimental SERS spectra of Ru(bpy)3

2+

excited at 355 and 405 nm on Ag and Al FONs (Figure S4).For both excitation wavelengths (355 and 405 nm), AgFONsperform better than AlFONS. Including these data points onour theoretical plots shows that the QSPP is less accurate at 355and 405 nm on AlFON 300 nm, where the experimental EFsare higher than those values predicted for pure Al (Figure 4D).For the Ag 170 nm sphere, we show that the theory matchesthe experimental EFs well at 355 nm, while on the Ag 280 nmsphere the experimental EF is higher than the predicted value.At 405 nm the EFs on Ag 170 and 210 nm are slightly less thanthose predicted by QSPP, while on Ag 280 the value issignificantly higher.For Ag, as the excitation wavelength drops below 400 nm,

the enhancement drops off rapidly, as is to be expected giventhat interband transitions (and therefore plasmon damping) isimportant below ∼325 nm. For Al, the EFs increase in goingfrom 200 to 400 nm and reach values greater than ∼105. InFigure 4A, there is also an important dip in the real componentof Al dielectric function near 800 nm that arises from interband

transitions, but this prediction of the theory has not been testedhere. From this analysis of the dielectric functions, it is clearthat Al provides consistent enhancement throughout the UV,whereas Ag provides strong enhancement in the visible thatdrops off rapidly below 350 nm. The QSPP provides goodagreement with the EFs observed for the 170 and 210 nmAlFONs (at λex = 229 nm), as well as for the 170 nm AgFONs(at λex = 355 nm), however this metric underestimates theenhancement observed for the 280 nm Ag and 300 nmAlFONs. These differences are likely due to nanoscale variationin the FONs, as well as near-field versus far-field effects,29 notcaptured by the simple QSPP EF metric.In conclusion, the present work demonstrates for the first

time fabrication of AlFONs and excitation of combinedresonance Raman and SERS at the deepest UV wavelength todate (λex = 229 nm). We confirm that AgFONs supportplasmonic enhancement at wavelengths greater than 400 nm,while below 400 nm we see only contributions from resonanceenhancement. We also demonstrate both experimentally andtheoretically that AlFONs provide electromagnetic enhance-ment in the 200−400 nm range. The magnitude of theenhancement is 103−105. However, when combined with theresonance effect, the total enhancement of the AlFONs is in the105−107 range, depending on the resonance enhancement ofthe molecule being probed, which is comparable to or greaterthan previously reported enhancement factors.7,9,19 Wedemonstrate that BPE on the AlFON 210 nm provides thelargest total enhancement at the deepest UV wavelength usedto excite SERS spectra to date.As this is the first report on the fabrication of AlFONs, there

is room for optimization of the substrates. We continue to workon characterizing various nanosphere sizes and depositionconditions to expand the use of AlFONs across a range of UVwavelengths. By combining the advantages of UV resonanceRaman with SERS, there is great potential for these FONs to beapplied in the collection of SERS spectra at ultralowconcentration, using label-free samples in a variety of fieldsincluding materials research, biological and chemical applica-tions, and catalysis.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.6b04296.

Raman instrumentation; substrate fabrication; nonreso-nant Raman spectra of mercaptohexanol; key factors toconsider for characterizing SERS substrates; enhance-ment factor calculations; relationship between EF andQSPP

2 for Ag and Al FONs incubated with BPE andadenine; experimental SERS spectra for Ru(bpy)3

2+

excited at 355 and 405 nm on Ag and AlFONs (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: vanduyne@northwestern.edu.

ORCIDBhavya Sharma: 0000-0003-4388-5702Michael B. Ross: 0000-0002-2511-0594Richard P. Van Duyne: 0000-0001-8861-2228

Figure 4. (A) The real (solid lines) and imaginary (dashed)components of the dielectric functions of Ag (black) and Al (blue).(B) QSPP

2 for Ag compared with the EM EF for benzenethiol onAgINRA (black inverted triangles; values from Greeneltch et al.44),with new experimental values for benzenethiol on AgFON excited at488 and 532 nm (red inverted triangles). Calculated EM EFs include10.5·QSPP

2Ag (gray) and QSPP2 for Ag (black). QSPP

2 for (C) Al (blue),and Al oxide (aqua) and (D) 10.5·QSPP

2Ag (gray), QSPP2 for Ag (black)

FONs fabricated from 170 (triangles), 210 (squares), 280 (circles),and 300 (diamond) nm sphere sizes with EFs for Ru(bpy)3

2+ fromexperiments included.

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Present AddressM.B.R. Department of Chemistry, Latimer Hall, University ofCalifornia, Berkeley, CA 94720−1460.Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript. B.S. and M.F.C. contributed equally.

FundingB.S., G.C.S., and R.P.V.D. acknowledge support from theNational Science Foundation Center for Chemical Innovationdedicated to Chemistry at the Space−Time Limit (CaSTL)Grant CHE-1414466. M.F.C., G.C.S., M.B.R., and R.P.V.D.acknowledge further support from the National ScienceFoundation Materials Research Science and EngineeringCenter (DMR-1121262). A.Z. and R.P.V.D. acknowledgeadditional support from a National Science Foundation SingleInvestigator Grant CHE-1506683. S.V.B, D.P., and S.A.A. weresupported by DTRA HDTRA1-15-1-0038. B.S. acknowledgessupport from the University of Tennessee at Knoxville (start-upfunds).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The scanning electron microscopy imaging was performed atthe Northwestern University EPIC facility (NUANCE Center),which has received support from the MRSEC program (NSFDMR-1121262) at the Materials Research Center, the Nano-scale Science and Engineering Center (NSF EEC-0647560) atthe International Institute for Nanotechnology, and the State ofIllinois, through the International Institute for Nanotechnology.

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Nano Letters Letter

DOI: 10.1021/acs.nanolett.6b04296Nano Lett. 2016, 16, 7968−7973

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