Modulation of Cu2–xS Nanocrystal Plasmon Resonance through

Modulation of Cu2−xS Nanocrystal Plasmon Resonance through Reversible Photoinduced Electron Transfer Rabeka Alam,† Molly Labine,†,§ Christopher J. Karwacki,‡ and Prashant V. Kamat*,† †

Radiation Laboratory and the Department of Chemistry & Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Edgewood Chemical Biological Center, U.S. Army Research, Development and Engineering Command, 5183 Blackhawk Road, APG, Aberdeen, Maryland 21010, United States S Supporting Information *

ABSTRACT: Copper sulfide (Cu2−xS) nanocrystals with nonstoichiometric composition exhibit plasmon resonance in the near-infrared region. Compositional changes and varying electron density markedly affect the position and intensity of the plasmon resonance. We report a photochemically induced phenomenon of modulating the plasmon resonance in a controlled fashion. As photogenerated reduced methyl viologen radicals transfer electrons to Cu2−xS in inert solutions, we observe a decrease in localized surface plasmon resonance (LSPR) absorbance at 1160 nm. Upon exposure to air, the plasmon resonance band recovers as stored electrons are scavenged away by oxygen. This cycle of electron charge and discharge of Cu2−xS nanocrystals is reversible and can be repeated through photoirradiation in N2 saturated solution followed by exposure of the suspension to air. The spectroscopic studies that provide mechanistic insights into the reversible charging and discharging of plasmonic Cu2−xS are discussed. KEYWORDS: plasmon resonance, copper sulfide, semiconductor nanocrystals, electron transfer, methyl viologen

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conductors are that size, shape, doping, temperature control, and phase transition can tune their free carrier concentration.3,7−13 This in turn allows modulation of the LSPR or switching on/off of the LSPR within a working device through applied potential.14,15 Luther et al. reported the LSPR frequency dependence on free carrier densities and the effect of doping in plasmonic quantum dots.3 Semiconductor nanocrystals with carrier concentrations of 1016 to 1021 cm−3 exhibit LPSRs in the near- to mid-IR spectral region, thus, making them useful in a wide range of applications including near field infrared imaging, lithography, optoelectronics, photothermal therapy, plasmon enhanced absorption for photon harvesting in the red end of the solar spectrum, and surface enhanced Raman spectroscopy in the near-IR region.16−19 Cu2−xS exhibits LSPR in the near-IR region (900−1800 nm), the exact position of which is determined by the ratio of Cu:S.20 Early studies have probed the effect of different electron donors or acceptors on the position and intensity of the LSPR.8,21 For

anoparticles of several different metals (Au, Ag, or Cu) exhibit single or multiple localized surface plasmon resonance (LSPR) bands in the visible spectrum. The location of this LSPR band is directly dependent on the nanoparticle’s chemical composition, size, and morphology. Plasmon absorbance that arises from the collective oscillation of charged free carriers, specifically electrons, is readily initiated by light excitation. Plasmonic metal nanoparticles have demonstrated to be useful in a multitude of applications including sensors, nanomedicine, catalysis, and energy conversion.1,2 LSPR is not just limited to metal nanostructures but also occurs in semiconductor nanocrystals with substantial free carrier concentrations. Given the low charge carrier density of semiconductor nanocrystals, one observes their plasmon resonance in the IR region. Recent studies have focused on exploring LSPR of vacancy doped copper chalcogenides (Cu2−xY; x > 0, Y = S, Se, Te), ZnO, and Si nanostructures.3,4 Cu2−xS is a p-type semiconductor that exhibits stoichiometry dependent bandgap, thus making this class of nanocrystals appealing to diverse fields.5,6 Nonstoichiometric Cu2−xS exhibit LSPR in the NIR spectral region due to their charge carrier density levels. Key advantages of using plasmonic semi© XXXX American Chemical Society

Received: December 21, 2015 Accepted: February 6, 2016

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Figure 1. (A) TEM micrograph of Cu2−xS nanocrystals with an average diameter of 13.7 ± 3.2 nm and (B) the corresponding absorption spectrum of 20 nM Cu2−xS in toluene, with a plasmonic absorbance at 1160 nm. (The instrument noise around 1650 is suppressed for spectral clarity.)

Figure 2. (A) Transient absorption spectra recorded (0−1000 ps) following 387 nm laser pulse excitation of 20 nM Cu2−xS nanocrystals in toluene and (B) the bleaching recovery monitored at 1160 nm (inset shows the recovery during first 10 ps).

example, Jain et al. showed that the electronic structure of these nanocrystals can be altered by addition of electron donors, e.g., sodium biphenyl leading to filling of copper vacancies or introducing electron acceptors like iodine which favors the creation of copper vacancies, resulting in holes in the valence band of copper(I) sulfide.21 Varying the concentration of copper ions in nonstoichiometric Cu2−xS nanocrystals allows tuning of LSPR as it introduces p-type vacancy. Xei et al. demonstrated that addition of Cu+ ions to Cu2−xS nanocrystals results in the filling-up of Cu vacancies leading to the decrease of the plasmon absorbance in a controlled manner.6

(Figure 1B) shows an onset of absorption around 520 nm, which corresponds to the bandgap transition (Eg = 2.38 eV). The prominent broad absorption band in the near-infrared with maximum around 1160 nm arises from the LSPR of Cu2−xS nanocrystals. This characteristic plasmonic absorption of semiconductor nanocrystals can be tuned by varying composition, charge carrier density, and/or relative ratio of Cu to S in the nanocrystals. X-ray photoelectron spectroscopy (XPS) measurements indicated the Cu:S composition of these nanocrystals to be 1.6:1 (or Cu1.6S). (The data of XPS measurements are shown in Supporting Information Figure S3.) The response of the plasmon band to direct bandgap excitation was probed by exciting the Cu2−xS crystals with 387 nm laser pulse in a femtosecond transient absorption spectrometer (fsTAS). Figure 2A shows the time-resolved difference absorption spectra recorded following 387 nm laser pulse. As we subject these nanocrystals to bandgap excitation, we expect an increase in the charge carrier density.22−25 Ultrabandgap excitation (387 nm) also causes generation of hot carriers. Collectively, these effects cause disappearance of the plasmon absorption band. The bleaching of this infrared band recovers quickly as the charge carriers thermalize and/or recombine to regenerate the LSPR of parent nanocrystals.26,27 The bleach recovery exhibits two distinct decays with a fast recovery occurring within 2 ps and the rest recovering in nanosecond time scale. Analysis of the bleaching recovery with a biexponential kinetic fit shows the short component yields lifetimes (τ1) of 0.85 ps and (τ2) 270 ps, respectively (Figure 2B). This shows that thermalization of photoexcited electrons

RESULTS AND DISCUSSIONS The low electron density of semiconductors as compared to metals makes them extremely sensitive to local environments. The dependence of LSPR on the electron density can offer new ways to design optical windows and sensors. Since the plasmon resonance in Cu2−xS is sensitive to electron density, we wanted to see whether transfer of electrons from photochemically generated electron donor could modulate LSPR. Optical Properties of Cu 2−x S Nanocrystals. We synthesized Cu2−xS according to the method described by Xei et al.6 Briefly, a mixture of sulfur, oleylamine, and octadecene is degassed and heated to 130 °C, followed by the addition of copper(I) chloride and heating to 200 °C (see Methods for detailed procedure). Transmission electron microscopy revealed Cu2−xS nanocrystals with an average diameter of 13.7 ± 3.2 nm (Figure 1A, Figure S1 shows size distribution histogram). The UV−vis absorption spectrum B

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changes recorded following the incremental addition of MV+• to a solution of 20% ethanol and 80% toluene in the absence and presence of Cu2−xS nanocrystals are shown in Figure 3, panels A and B, respectively. The magnitude of absorbance in Figure 3A provides an estimate of the net concentration of MV+• corresponding to each incremental addition to the sample in the absence of Cu2−xS. The residual MV+• absorption after its equilibration with Cu2−xS (Equilibrium 1) shows the amount of unreacted component. Thus, the absorbance difference between spectra A and B (Figure 3) represents electrons transferred to Cu2−xS.

and charge carrier recombination in Cu2−xS nanocrystals is an ultrafast process occurring in the subpicosecond time scale accounting for 70% of the bleaching recovery. The remaining bleach recovery is seen within the nanosecond time scale. The two distinct kinetic processes parallel the fast bleaching recovery of thermalization of photoexcited electrons followed by slower phonon−solvent interactions in metal nanoparticles.28,29 The obvious question then is whether one can modulate the plasmon resonance through externally generated electron donor medium. Electron Transfer between MV+• and Cu2−xS. Methyl viologen (MV2+) has a relatively low reduction potential (−0.45 V vs NHE) and is often utilized to probe the electron transfer processes at semiconductor and metal nanoparticle interface.30−32 For example, MV2+ can be reduced with semiconductor nanoparticles like TiO2 and CdSe/CdS quantum dots, as well as gold metal clusters.24,25 MV2+ is a colorless solution with an absorbance maximum at 260 nm (ε = 18 300 M−1 cm−1), and its reduced form of methyl viologen radical (MV+•) exhibits characteristic absorption at 395 nm (ε = 41 100 M−1 cm−1) and at 611 nm (ε = 13 800 M−1 cm−1).33,34 It has also been demonstrated that in the presence of an electron acceptor, MV+• also has the potential to reduce metal ions to metal nanoparticles.32 MV+• in the present experiments was generated photochemically by subjecting an ethanolic solution to UV irradiation.34 Ethanol serves as a reducing agent to quench the excited methyl viologen (MV2+)* and produce blue colored MV+• radicals.32−34 The MV+• radicals, which are stable in an inert atmosphere, can transfer electrons to nonstoichiometric Cu2−xS nanocrystals. The transfer of electrons is likely to fill the vacancies in the valence band (Scheme 1). The absorption

y MV +• + Cu 2 − xS → (y − z)MV +• + z MV2 + + Cu 2 − xS(z e)

(1)

An interesting observation in Figure 3B is the decrease in plasmon absorbance (1160 nm) with each incremental addition of MV+• to Cu2−xS suspension. As the electrons are transferred to Cu2−xS, the p-type vacancies are filled and we see a decrease in the LSPR. Similar changes in the plasmon resonance have been observed upon introduction of a reducing agent to semiconductor nanocrystals.21 Another observation in this experiment is the shift in absorption peak to lower energy region upon addition of the electrons. This red-shift in LSPR to lower energy is opposite to blue-shift in LSPR seen with the increased electron density in Au and Ag nanoparticles.35,36 We attribute this red-shift of the LSPR peak to the changes in the valence state of Cu as the electrons are transferred to Cu2−xS. Similar red-shifts in LSPR was also reported for Cu2−xS nanocrystals being subjected to electron donating species.6,18,26,37,38 On the basis of these experiments, we can further infer that the LSPR is sensitive to the filling-up of p-type vacancies as well as oxidation state change of Cu2−xS. We further estimated the electrons transferred to Cu2−xS from the number of MV+• species consumed following each addition (Figure 3A,B). Note that only a fraction of MV+• transfers electrons as the two systems attain Fermi charge equilibration. This analysis allowed us to obtain a quantitative estimate of the dependence of decreased plasmon resonance on the increase in electron density. Figure 4A shows the relationship between the decrease in LSPR and MV+• added to the Cu2−xS suspension, derived from Figure 3. The detailed analysis of the estimation is presented in Table 1. After the transfer of 23 nmol of electrons (a value based on MV+• consumed) to 0.060 nmol of Cu2−xS, a saturation in the

Scheme 1. Electron Transfer from MV+• to Cu2−xS Process under Inert Atmosphere Results in the Quenching of Plasmon Resonance in Cu2−xS

Figure 3. Absorption spectra of MV+• (A) and MV+• plus 20 nM Cu2−xS (B) at different concentrations (concentration of MV+• calculated from the absorbance monitored at 611 nm using ε = 13 800 M−1 cm−1). All spectra were collected in 20% ethanol and 80% toluene mixture. C

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Figure 4. (A) Plot of electrons transferred vs MV+• added to 0.06 nmol of Cu2−xS in a mixture of 20% ethanol and 80% toluene. Saturation of electron transfer is observed once 23 nmol of electrons is transferred to the Cu2−xS nanocrystals. (B) Plot of ΔA vs electron transferred.

Table 1. Electron Equilibration between Cu2−xS and MV2+/ MV+• and Determination of Apparent Fermi Levela

Scheme 2. Fermi Level Equilibration between Cu2−xS Nanocrystals and MV2+/MV+• Redox Couple

equilibrium concentration (nmol) MV+• added (nmol)

[MV+•]

Cu2−xS (etransfer)

apparent Fermi level EF (V vs NHE)

0 9.6 15.0 24.6 44.1 62.7 84.6 120.9

0 1.5 5.1 12.0 24.9 39.9 60.6 98.4

0 8.1 9.9 12.6 19.2 22.8 24.0 22.5

−0.41 −0.43 −0.45 −0.45 −0.46 −0.47 −0.49

with MV 2+/ MV +• redox couple. By substituting the concentration values at equilibrium in reaction 2, we obtain the EF in the range of −0.41 to −0.49 V vs NHE corresponding to the addition of 1.5−98.4 nmol of MV+•. It is interesting to note that the EF shifts to more negative potential as we increase MV+• concentration. With increased number of electrons getting transferred to the Cu2−xS, we expect to see the apparent Fermi level to become more negative, finally attaining a value close to the conduction band of the semiconductor. On the basis of this analysis, we estimate the conduction band of Cu2−xS to be around −0.5 V. This indirect estimate of the apparent conduction band energy using a redox couple mediated electron equilibration provides energetics of conduction and valence bands of Cu2−xS for further evaluation in photocatalytic reactions. The Reversibility of Electron Transfer. If indeed the electron equilibration (between Cu2−xS and MV2+/MV+•) hypothesis is valid, it should be possible to revert back the LSPR by decreasing the concentration of MV+•. We checked this reversibility by subjecting an equilibrated system to photoirradiation in N2 atmosphere followed by the exposure to air. The MV+• radical is sensitive to the presence of oxygen as it reverts back to its parent MV2+ form. At the same token, the electrons transferred to Cu2−xS are also susceptible to the scavenging by dissolved oxygen, thus, decreasing the net electron concentration in air. Spectra a and b in Figure 5A show the absorption of Cu2−xS before and after addition of MV+• radicals in N2 atmosphere. As expected, the plasmon band disappears with increased presence of MV+• in the system. Upon exposure to air, however, the plasmon band recovers, thus, confirming the reversibility of electron transfer across Cu2−xS nanocrystals (Figure 5A(c)). To check the reversibility of this electron transfer, we employed plasmon response at 1160 nm as a probe. A solution containing both Cu2−xS and MV+• was deaerated with N2 and

a

Known amount of photochemically reduced MV2+ (MV+•) was added to 0.06 nmol of Cu2−xS and equilibrium concentrations were measured from the absorption spectra. EF was determined from reaction 2 and using E0 (MV2+/MV+•) = −0.45 V.

electron transfer could be seen. Additional changes in the plasmon absorbance of Cu2−xS could not be seen at higher concentration of MV+•. Figure 4B shows the linear relationship between the decrease in absorbance versus the electron transferred to Cu2−xS. On the basis of this data, we estimate a transfer of about 380 electrons per Cu2−xS nanocrystal. It is interesting to note that this value is similar to the estimated number of vacancies reported in a previous study.3 Given the low carrier density of the Cu2−xS nanocrystals, such a large capacity to accept electrons seems reasonable. Fermi Level Equilibration. It is apparent that MV2+/ MV+• redox couple attains equilibrium with the Cu2−xS system following each addition of MV+• (Reaction 2). Such electron equilibration between semiconductor nanoparticles and redox couple provides a convenient means to determine the apparent Fermi level (EF) of the system using Nernst equation. E F(Cu 2 − xS) = E(MV2 +/MV +·) = E 0(MV2 +/MV +•) − 0.059 log[MV +•]/[MV2 +]

(2)

With increasing electron transfer, the apparent Fermi level of Cu2−xS shifts to more negative potentials. In the present study, we further analyzed the equilibrium concentrations of MV2+ and MV+• to obtain the apparent Fermi level (EF) of the Cu2−xS. From Table 1, we obtain the concentrations of MV2+ and MV+• following the equilibration with Cu2−xS. Scheme 2 shows the energy levels of Cu2−xS before and after equilibration D

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Figure 5. Reversibility of charging and discharging Cu2−xS nanocrystals in the presence of MV+•. (A) The absorption spectra of the deaerated Cu2−xS suspension (a) before and (b) after equilibration with MV+•. (c) Absorption spectrum of the same solution upon exposure to air. (B) The reversibility of electron transfer as probed through LSPR response. The absorbance change at 1160 nm was monitored following the UV exposure of N2 saturated solution followed by exposure to air: (a) Cu2−xS and (b) Cu2−xS and MV2+ (Absorption spectra corresponding to these measurements are shown in Figure S4 in the Supporting Information). All experiments were conducted using 20 nM Cu2−xS and 62 μM MV2+ in a solution of 20% ethanol and 80% toluene.

then irradiated with UV light. As indicated above, MV2+ undergoes reduction when UV-irradiated in the presence of ethanol. As the photogenerated MV+• radicals transfer electrons to Cu2−xS, we observe a disappearance of 1160 nm absorption. When this solution is exposed to air, we see a recovery of the plasmon absorption. We repeated sequential irradiation (in N2 atmosphere) and exposure to air for several cycles. The reversibility of the plasmon absorption to the cycle of electron injection and removal can be visualized through the results presented in Figure 5B. As confirmed through transient absorption experiments, direct excitation of Cu2−xS does not produce long-lived charge carriers to sustain electronic storage, and hence, we do not see changes following direct excitation with visible light (>400 nm). In addition, we see no plasmon absorbance change when we exclude MV2+ from the solution (Figure 5B(a)). Transferring electrons to Cu 2−xS and modulating the plasmon resonance in a controlled manner through electron transfer is a valuable approach for designing sensitive probes and optical windows in the IR region.

monochromator utilizing PMT and PbS detectors with range of 190−3200 nm. Transmission Electron Microscopy (TEM). TEM measurements were performed on a FEI Titan 80−300 transmission electron microscope (80-300 kV, 0.19 nm resolution) with Gatan 4 × 4 CCD detector (NDIFF). Samples were deposited onto Ni TEM grids. Particle sizes were analyzed manually with statistical analysis performed using ImageJ software on populations of at least 100 counts. Thermal Gravimetric analysis (TGA). TGA measurements were preformed on Mettler Toledo TGA/DSC-1. TGA was equipped to analyze samples from 25−1600 °C with submicrogram resolution over the whole measurement range. Samples were run under N2 flow from 25 to 600 °C. X-ray Photoelectron Spectrometer (XPS). XPS analysis of Cu2−xS nanocrystals was preformed using a PHI VersaProbe II, equipped with monochromatic X-ray beam focusable to 9 μm spot size. Samples for XPS were prepped by drop casting colloids onto fluorine doped tin oxide glass. Xe Lamp. Photoirradation of samples was preformed with an Oriel 300 W Xe lamp. Samples were sealed in cuvettes and purged with nitrogen for 15 min prior to irradiation. A water filter was used to remove strong UVs. Femtosecond Transient Absorption Spectrometer (fsTAS). Transient absorption measurements were performed using a Clark laser with a 775 nm fundamental pulsed at 1 kHz with 130 fs fwhm pulse durations. The ultrafast system was used operated with Helios software. The fundamental is split to generate a nIR light probe by focusing through a Ti:sapphire crystal. For 387 nm pump excitation, the second harmonic of the 775 nm is generated and used to excite samples. Transient absorption spectra are recorded as a difference between probe signals with/without a pump pulse, and the delay between pump and probe is controlled to generate spectra at varied times following excitation. The kinetics data was fit using Igor pro fitting software. Synthesis of Cu2−xS. Cu2−xS nanocrystals were prepared following the heat-up method described by Xei et al. with some modifications.6 First, a sulfur solution was prepared by dissolving 2 mmol of S in 5 mL of oleylamine and 5 mL of octadecene in a 25 mL three-neck roundbottom flask at 130 °C under vacuum. Next, the clear yellow mixture was cooled to room temperature under argon flow, and 1 mmol of CuCl was added to the sulfur mixture. The flask was then placed under vacuum at room temperature for 30 min to remove oxygen, followed by heating to 200 °C for 10 min. The resulting Cu2−xS nanocrystals were dark green in color. The final solution was cooled to room temperature, and excess ligands and unreacted precursors were removed via centrifugation in a mixture of toluene and ethanol. Purified product was dispersed in toluene, and size segregation was

CONCLUSION In summary, we have succeeded in modulating LSPR of Cu2−xS nanocrystals through photochemically generated MV+• radicals. The electron transfer between Cu2−xS and MV2+/MV+• is reversible as probed through the changes in the infrared LSPR absorption. The possibility of controlling LSPR band through a photochemically generated electron donor can be further extended to create an optical window in the infrared. Opportunity also exists to utilize LSPR of Cu2−xS nanocrystals to influence a semiconductor assisted photocatalytic reaction. Indeed, the apparent Fermi level of the Cu2−xS nanocrystals as measured through electron equilibration shows favorable energetics for inducing such photocatalytic reactions. METHODS Chemicals and Materials. Copper(I) chloride (CuCl, 99.99%), sulfur (S, 100 mesh, 99/99%), oleylamine (70%), octadecene (95%), methyl viologen (MV2+, 98%), toluene, and anhydrous ethanol were purchased from Sigma-Aldrich. Instrumentation. UV−Vis−Near-IR Spectrophotometry. The UV−vis-near-IR absorbance spectroscopy measurements were collected on a Jasco V-670 spectrophotometer between 300 and 2000 nm. The instrument is equipped with a double-beam, dual grating E

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ACS Nano preformed by centrifugation at 1000 rpm to remove large nanocrystals. The supernatant was removed and stored in a glovebox to limit exposure to oxygen. Nanocrystal Size, Concentration, and Stoichiometry Calculation. Cu2−xS nanocrystals particle sizes were analyzed manually with statistical analysis performed using ImageJ software on populations of at least 100 counts of TEM images. Particles size histogram is shown in Figure S1, showing an average diameter of 13.7 ± 3.2 nm. The concentration of the Cu2−xS nanocrystals was estimated using TGA (Figure S2) and size distribution from TEM. Repeated TGA experiments showed 49.8% mass loss before reaching 380 °C, which accounts for Cu2−xS nanocrystals. With the use of the mass remaining and the mass of a nanocrystals (calculated from the volume and density of CuS and Cu2S), the concentration was estimated. With a remaining mass of 0.195 mg, a concentration of 600 nM was calculated for the stock Cu2−xS sample in toluene. The ratio of Cu:S in Cu2−xS nanocrystals used was measured to be 1.6:1 (Cu1.6S) using XPS, as shown in Figure S3. Electron Transfer Experiment. All experiments were preformed using 20 nM Cu2−xS and 62 μM MV2+ in a solution of 20% ethanol and 80% toluene. Samples were purged with N2 for 15 min prior to light exposure.

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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/acsnano.5b08066. Experimental details that include instrumentation, calculations, and characterization of Cu2−xS (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Address §

Coop Student, University of Guelph, Guelph, ON N1G 2W1, Canada Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We would like to acknowledge Steven Kobosko and Ian Lightcap for help with XPS data collection and analysis. We thank the ND Energy Materials Characterization Facility (MCF) for the use of the UV−visible−Near-IR spectrometer, XPS, and TGA. The Sustainable Energy Initiative (SEI) funds MCF, which is part of the Center for Sustainable Energy at Notre Dame (ND Energy). We also thank Notre Dame Integrated Imaging Facility (NDIIF) for electron microscopy facilities. P.V.K. and R.A. thank the Army Research Office for the support through the award ARO 64011-CH. P.V.K. and M.L. also acknowledge the support by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DEFC02-04ER15533. This is a document number 5099 from the Notre Dame Radiation Laboratory. REFERENCES (1) El-Sayed, M. A. Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes. Acc. Chem. Res. 2001, 34, 257−264. (2) Faucheaux, J. A.; Stanton, A. L. D.; Jain, P. K. Plasmon Resonances of Semiconductor Nanocrystals: Physical Principles and New Opportunities. J. Phys. Chem. Lett. 2014, 5, 976−985. F

DOI: 10.1021/acsnano.5b08066 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.5b08066 ACS Nano XXXX, XXX, XXX−XXX