Ag Interface of a Large Area Nanoarchitectured Ag

Article pubs.acs.org/JPCC

Study of the C60/Ag Interface of a Large Area Nanoarchitectured Ag Substrate Using Surface-Enhanced Raman Scattering Akram A. Khosroabadi,†,§ Dallas L. Matz,‡,§ Palash Gangopadhyay,*,† Jeanne E. Pemberton,‡ and Robert A. Norwood*,† †

College of Optical Sciences, The University of Arizona, Tucson, Arizona 85721, United States Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States

‡

ABSTRACT: Plasmonic Ag nanopillars have been fabricated and used as a surfaceenhanced Raman scattering (SERS) substrate. The effective surface area of the sample is determined using underpotential deposition (UPD) of thallium and agrees well with a geometrical calculation using ImageJ analysis of SEM images. In order to find the SERS enhancement factor (SEF), a similar sample is coated with Pt, which shows no plasmon response at the excitation wavelength of 532 nm. SEF values on the order of 105 are obtained for Ag nanopillar substrates. Several monolayers of C60 were deposited on these Ag nanopillars, and the Raman spectral results indicate charge delocalization at the interface between C60 and Ag. FDTD simulation of the electric field confirms the experimental results; on the basis of these simulations, the electric field modulates with increasing diameter of the pillars, while the pitch (center-to-center distance) is fixed at 200 nm.

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INTRODUCTION Since its discovery, SERS has drawn substantial attention due to its potential to overcome the low sensitivity that plagues traditional Raman spectroscopy.1 SERS not only improves the surface sensitivity but also facilitates the study of various interfacial processes by enhancing the Raman scattering from analytes on metal/semiconductor surfaces.2,3 With potential applications in fields ranging from plasmonics to diagnostics,4 the SERS effect is predominantly an electromagnetic effect arising from an increase in the local optical field due to multiplicative amplification of the excitation laser and the reradiated Raman scattered light.5 This optical enhancement is commonly associated with the excitation of surface plasmon oscillations in most SERS systems.5 Nanostructured metal/ metal oxide surfaces often lead to surface plasmon resonance formation and a coupling between the localized surface plasmon polaritons (SPP) and electromagnetic radiation incident on the substrate surface resulting in intense absorption in the near-IR and visible-near UV region and enhancing the Raman scattering signal intensity by many orders of magnitude.6−8 For surface nanofeatures smaller than the incident optical wavelength, the surface plasmon resonance normal modes of oscillation are resonant with both the excitation and scattered photons.9 The frequency of the surface plasmon resonance depends on the dielectric constant of the metal/metal oxide and the dimensions of the nanofeatures which are responsible for the SERS effect. The SERS intensity decreases significantly with nanostructures that are either significantly larger than ∼100 nm or smaller than ∼10 nm.10 However, SERS enhancement depends greatly on the geometrical configuration of the nanostructures and their interstructure interactions. The fact that the nanostructure © 2014 American Chemical Society

plasmon resonance allows direct coupling of light to the resonant electron plasmon oscillation has spurred tremendous efforts in the design and fabrication of highly enhancing substrates based on nanostructured films and metallic nanoparticles in both engineered and random arrays.11 The most established substrates are those that are sprayed with Ag or Au colloids, resulting in intense SERS signals at the narrow junctions between the particles. Junctions between aggregated nanoparticles are believed to be SERS “hot spots” where large field enhancements allow for single molecule detection in some cases. Although spraying Au or Ag colloids on a substrate provides extremely high enhancement factors at local hot spots, it has thus far been difficult to achieve reliable, stable, and uniform SERS signals spanning a wide dynamic range on large area substrates using this method.12,13 Furthermore, such substrates suffer from limited stability and reproducibility and, in general, are not amenable to large-scale production of SERSbased sensors.14 More reliable and uniform surface enhancements are expected from substrates containing anisotropic nanostructured plasmonic materials. Anisotropic metallic nanostructures allow: (1) tunable plasmon absorption bands that can be achieved by adjusting the nanostructure aspect ratio and separation to be in resonance with common laser radiation sources used for Raman excitation in order to optimize the electromagnetic enhancement mechanism;15 (2) symmetry breaking leading to more complex plasmon propagation, potentially giving more intense electromagnetic field generation from the structure and in gaps formed between these Received: May 30, 2014 Revised: July 3, 2014 Published: July 10, 2014 18027

dx.doi.org/10.1021/jp505364d | J. Phys. Chem. C 2014, 118, 18027−18034

The Journal of Physical Chemistry C

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Figure 1. (A) SEM micrograph of a 50 nm Ag-coated nanopillar (inset shows a top view of the same image). (B) Absorption spectra of 50 and 10 nm Ag deposited on PAN nanopillars. Inset shows a schematic representation of the Ag-coated nanopillar cross section. Deposition of 10 nm Ag on the pillars creates small islands of Ag on the pillars. Absorption peaks for 50 nm Ag are much more pronounced than those for 10 nm Ag-coated pillars.

materials;16 (3) designer assembly of various nanostructure formations that enables analyte molecules to adsorb preferentially in the fractal space between nanostructures or in SERS “hot spots”, giving rise to large field enhancement;17 and (4) anisotropic nano-objects that have shown interesting size and shape-dependent properties, thus motivating interest in their controlled assembly into functional architectures for SERS. Previously, we have shown that such large area metal and metal oxide nanostructures can be fabricated in a reproducible way using a modified nanoimprinting technique.18−20 This work extends the use of these substrates for SERS experiments and demonstrates their use on deposited layers of C60, which is an important interface in organic optoelectronic devices. Fabrication of the SERS substrates using our novel nanoimprint technique is relatively easy, fast, and reproducible, which makes it a great fabrication tool for this purpose.

Planar reference samples are fabricated under the same conditions in the absence of nanopillars.20 In order to compare the SERS response of the pillars with different dimensions, 10 and 50 nm of Ag are deposited on the PAN pillars. At or below 10 nm thickness of Ag, the sample is not coated uniformly18 and PAN pillars are covered with noncontinuous islands of Ag. Hence, the Raman signal is not reproducible, and due to noncontinuity of the sample, we were not able to accurately determine surface area via UPD on 10 nm sample. Scanning electron microscopy (SEM) images of the samples (Figure 1a) are used to find the geometrical surface area enhancement factor (SAE), which is the ratio of nanostructured sample area compared to a planar reference one. Using a topdown view SEM image, one can measure the diameter and area fraction of the pillars. The height of the pillars is measured using a profile view SEM image. ImageJ was used to process the images and calculate SAE. The effective surface area of the silicon mold containing the nanopillars is 1 cm2, yielding a total number of nanopillars of ∼2.5 × 109. A variety of different materials can be deposited onto the pillars. With dependence on the thickness of the deposited material, SAE will change, reaching a value as high as 11 in some structures.19 In our SERS study, the effective area of the sample was 0.7 cm2 with ∼1.78 × 109 pillars. The schematic in Figure 1b shows the dimensions of the Ag nanopillars used to calculate the total area of the sample. If we assume that the diameter and height of each pillar are d and h, respectively, and that the effective surface of the sample has a diameter of D, the total surface area of the sample, including side walls of the pillars, is then given by A = πD2/4 + N*(πdh) = 2.18 cm2, showing 3.11 times the enhancement in the surface area compared to a planar sample. Here, N refers to the number of pillars. Surface Plasmon Resonance (SPR) of Ag Nanopillars. Substrates with Ag nanoparticles of 25 to 103 nm diameter have been previously reported. These substrates were fabricated by depositing thin layers of Ag of different thicknesses. The SPR wavelength for these substrates can be tuned from 428 to 606 nm, depending on the substrate, dimensions, and annealing temperature.21 Figure 1b shows absorption spectra for nanopillars coated with 10 and 50 nm of Ag. The absorption peak at 602 nm is due to interaction between the pillars. The smaller and broader peak at 475 nm is the SPR frequency of individual Ag nanopillars.

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EXPERIMENTAL SECTION Materials. H2SO4 (98%, EMD), HClO4 (Macron Chemicals, 70%), Na2SO4 (Macron Chemicals, 99.7%), and Tl2SO4 (Sigma-Aldrich, 99.995%), PAN (Mw 150000, Sigma-Aldrich), DMF (Sigma-Aldrich, 99.9%), and C60 (Sigma-Aldrich, 99.5%) were used as received. Electrolyte solutions were made in water (>18 MΩ resistivity, 99%) was purified by vacuum distillation prior to being made into a 5 mM solution in ethanol (Decon Laboratories, 200 proof). The Ag target is 99.99% pure (Kurt J. Lesker). Thiophenol monolayers were made by immersion of the metal (Ag and Pt)-coated PAN on glass substrates for 1 h followed by rinsing with ethanol. Ag Nanopillar Fabrication. Nanopillars of polyacrylonitrile (PAN) are fabricated on glass substrates using a modified nanoimprinting technique (NIMP).18 A 15 wt % solution of PAN in dimethylformamide (DMF) is prepared using an oil bath at 165 °C under stirring at 300 rpm. PAN is spun-cast onto a silicon mold consisting of nanoholes 120 nm in diameter, 250 nm in depth, and on a 200 nm center-to-center pitch. After removing the residual solvent from the PAN by annealing at 150 °C, the pillars are then delaminated from the mold and transferred to a glass substrate, which is covered by a planar layer of PAN. Different thicknesses of Ag are deposited on the pillars using the DC sputtering method, with a deposition rate of 1.7 Å/s and argon as the working gas. 18028



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Figure 2. (A) xy and (B) xz distribution of plasmonic E-field amplitude on the pillars when excited with light incident at an angle 30° relative to the x axis. The false color bar represents corresponding E-field amplitude.

Figure 3. (A) One-dimensional (1D) representation of E-field intensity at different regions of the sample at y = 0 (xz plane) and (B) time dependence of the electric field at various points on the nanopillars coated with 50 nm thick Ag.

Electrochemistry. Surface areas of Ag- and Pt-coated nanopillars were determined using cyclic voltammetry. A BAS 100 W electrochemical workstation (Bioanalytical Systems, Inc.) was used with a Pt wire loop as the counter electrode and an Ag/AgCl reference electrode. A PTFE sandwich cell was used, which provides a 0.713 cm2 geometric area for the working electrode. The absolute surface area of PAN coated with 50 nm of Ag was determined following procedures outlined previously from this laboratory.22 Ag-coated PAN surfaces were used as the working electrode and were cycled at 100 mV/s between −200 and −750 mV vs Ag/AgCl in 0.75 mM Tl2SO4/0.25 M Na2SO4/1 M HClO4. The amount of charge passed upon Tl monolayer underpotential deposition is measured by integration of the stripping wave. Surface area is calculated using 136 μC/cm2 for Tl monolayer deposition as reported by Bewick and Thomas.23 Surface areas of these Agcoated PAN nanopillars were determined to be 2.04 ± 0.35 cm2, in good agreement with the estimates above. In a similar manner, the surface area of Pt-coated PAN nanopillars can be determined using the charge passed during hydrogen chemisorption. For this, Pt-coated substrates were cycled at 100 mV/s between −200 and +1200 mV versus Ag/AgCl in 1 M H2SO4. The charge passed upon monolayer hydrogen adsorption was converted to a surface area using 226 μC/cm2 as reported by Hubbard et al.24 Surface areas measured on these Pt-coated PAN nanostructures were 2.12 ± 0.61 cm2. Raman Spectroscopy. An Excelsior diode-pumped laser at 532 nm (Spectra-Physics) was used as the Raman excitation source with 13−150 mW of power at the surface (controlled using ND filters.) The exciting beam was incident on the substrate at 30° with respect to the xy plane, producing an ellipse of dimensions ∼0.4 mm × 0.2 mm. Scattered light was

collected at 60° from the xy plane using a Nikon 50 mm focal length, f/1.2 camera lens, and focused onto a 19 μm entrance slit of a single grating monochromator (HORIBA Jobin Yvon iHR 320) with a 1200 groove/mm grating blazed at 500 nm coupled to a Newton EM-CCD detector (Andor Technology DU971P−BV). Rayleigh scattered light was rejected with a 532 nm Razors Edge long wave pass filter (Semrock). Peak fitting of Raman spectra was performed with Grams 32 (Galactic) using a 100% Gaussian peak shape. Spectral peaks for C60 were fit on the basis of values reported by Dong et al.25 Peak frequencies were constrained to within ±5 cm−1 of these values, relative intensities to within ±20%, and fwhm values to within ±10 cm−1 of their reported peaks. Background amorphous carbon modes were fit using four broad Gaussian bands in a manner previously outlined by Ferrari and Robertson with peak frequencies constrained to ±20 cm−1 of their values, intensities to ±50%, and fwhm values to within ±50 cm−1.26,27 Fits were deemed acceptable for χ2 > 0.99.

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RESULTS AND DISCUSSION Finite Difference Time Domain Simulations. Finite difference time domain (FDTD) software, commercially available from Lumerical, was used to simulate the electric field enhancement in these nanopillar arrays. One nanopillar is simulated, with Bloch boundary conditions used in the x and y directions and a perfectly matched layer (PML) boundary condition in the z direction. As noted above, the 10 nm Ag coatings consist of islands on the PAN pillars; therefore, in the simulation, the PAN pillar is decorated with small Ag nanoparticles. In contrast, the sample with 50 nm Ag has a continuous Ag coating that is not conformal everywhere. The sidewalls of the pillars are coated with a thinner Ag layer than

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Figure 4. Distribution of plasmonic E-field amplitude on (A) xz and (B) xy (3D representation) planes of the pillars when excited with light incident at an angle 30° relative to the x axis.

Figure 5. (A) 1D representation of E-field intensity at different regions of the sample at y = 0 (xz plane) and (B) time dependence of the Ex field at various points on the nanopillars coated with 10 nm thick Ag. The E field fluctuates on top of the pillars due to hot spots from islands of Ag.

Figure 6. Change in plasmonic E field on top of Ag-coated pillars with varying thickness of Ag in the z direction (changing height, panel A) versus varying thickness of Ag in the x direction (changing radius, panel B).

discontinuity between the dielectric constants of the materials, as expected from the boundary conditions of Maxwell’s equations. Since the imaginary part of the dielectric constant of air is zero, light propagates a longer distance in air than in Ag or PAN. At the bottom of the pillars, the E field is more confined at the interface of PAN/Ag, whereas on the top of the pillars, light is more localized at the Ag/air interface. The time dependence of the electric field is shown in Figure 3b. Electric field propagation for the point on the top of the Ag pillar at point A is shown in black. As expected from Figure 2a), the electric field in the center of the pillar is smaller than at the

the target thickness, and the Ag is thicker on top of the pillars than in the spacing plane between them.19 Thickness and conformity around the nanopillars were confirmed by SEM images recorded on cross sections of Ag nanopillar samples. Figure 2 shows the simulated electric field distribution for 50 nm Ag nanopillars. The angle of incidence is 30°, and the electric field is in the x direction. The duration of the pulse is 3.51 fs. The existence of hot spots on the edges of the Ag is evident. There is no enhancement in the electric field component of the light inside the PAN layer. Extra features at the interface of PAN/Ag and Ag/air are due to the 18030



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intensities. From our simulation shown in Figure 2A, the electric field enhancement factor is |(Eloc)/(E0)|4, where ⟨Eloc⟩ is average E-field amplitude in the xy plane at the top of the pillar. Comparison of simulated E-field intensity between nanostructured and planar substrates, predicts a factor of 4.5 × 104 in Raman signal intensity enhancement in the nanostructured Ag substrate. Surface-Enhanced Raman Scattering. Figure 7 shows the Raman spectra of Ag-coated PAN nanopillars with (red

edge of the pillar (point C in blue). The blue line shows the change in the real part of the electric field in the x direction at the edge of the Ag pillar. Figure 3b shows evidence for oscillation of the electric field, and after a few femtoseconds the pillar is polarized, which is a result of surface plasmon propagation. The electric field then reaches its maximum at about 15 fs and then gradually decays. In between the pillars (Point B in the inset in Figure 3b), the electric field decays more slowly than at the top of the pillar. Furthermore, the field strength between the two pillars is larger than that on the top, indicating that there is strong interaction between the pillars. Small nanoislands on the 10 nm Ag coated pillars result in nonuniform distribution of hot spots on top of the pillars (see Figure 4, panels a and b). On the bottom of the pillars, light is more localized at the interface of Ag/PAN and extends in between the pillars due to noncontinuous islands of Ag at the base of the pillars. In order to better describe the electric field behavior in different parts of the sample, 1D plots of the E field are shown in Figure 5a. Peaks in the electric field on top of the pillar are due to various hot spots caused by Ag islands. There is strong enhancement in the electric field at the bottom of the pillar in the space between the pillars. Figure 5b shows the change in the real part of the timedependent electric field in the x direction for different regions of the pillar. On top of the pillar, the electric field oscillates and after reaching a maximum decays to less than half its initial value and then begins to increase until it reaches a secondary maximum and eventually decays. The distribution of hot spots on top of the pillar causes variations in the enhancement of the electric field. The simulated normalized electric field intensity versus the radius of the Ag pillars is shown in Figure 6a. The incident wavelength is fixed at 532 nm, as this was the wavelength used for excitation in our experimental Raman work. The polarizability of the pillar changes with radius and is weakly oscillatory. The SPR frequency of the structure is dependent on the geometry and dimensions of the pillars; as the radius increases, the SPR frequency red shifts as expected. Figure 6b shows the simulated normalized electric field at the interface of silver and air at a constant radius of 75 nm versus height of the pillar. The aspect ratio of the pillar affects the SPR frequency, which also depends on the polarization of the light. Furthermore, the incident wavelength should be in resonance with SPR to be able to excite it. With dependence on the polarization and angle of incidence of the light, the longitudinal frequency of surface plasmon resonance can be excited, which is attributed to the nanopillar length and its aspect ratio. The longitudinal surface plasmon frequency redshifts with increasing nanopillar length. The SPR frequency can be detected as a minimum in the reflection spectrum. With increasing nanopillar height, the SPR frequency will red or blue shift depending on the polarization. Further, with increasing height, the dips in the reflection spectrum broaden (data not shown). The enhancement factor (G) due to SPR can be expressed by the following formula28 G=

E loc(r , ω) E◦(ω)

Figure 7. Raman spectra of (a) Ag-coated PAN nanostructure without thiophenol ML, (b) thiophenol ML on Ag-coated PA, and (c) thiophenol monolayer on Pt-coated PAN nanostructures.

trace) and without (black trace) a thiophenol monolayer (ML), along with the spectrum from a thiophenol ML on Pt-coated nanopillars (blue trace). The most prominent features of the spectra from the Ag-coated substrates are the large broad bands centered near 1350 and 1580 cm−1. These bands are ascribed to the D and G bands, respectively, of amorphous carbon (aC).26,29,30 This a-C arises from carbonization of PAN at the Ag/PAN interface as previously reported by Xue and coworkers.31 The carbonization process has been ascribed to formation of an Ag−nitrile surface complex that activates the PAN leading to CN bond cleavage, eventually resulting in the production of coupled aromatic heterocycles that further degrade to a-C. Quantification of the surface-enhancement factor (SEF) for these surfaces is accomplished by normalization of the Ramanscattered signal levels of a vibrational band of an adsorbed thiophenol ML at the Ag-coated pillars to that from the Ptcoated pillars. This approach is predicated on the assumption of a unity enhancement factor for the Pt-coated substrates as previously described by Taylor.22,24,32 The ν(C−C) ring mode33 at 997 cm−1 was chosen for this analysis because of its high intensity and its lack of spectral overlap with the a-C background. The signal intensity (S) in surface-enhanced Raman scattering is given by22,34

S = βsDsADPDt ΩTQFf

(2)

where βs is the Raman scattering cross section of the adsorbate, Ds is the molecular number density of the adsorbate on the surface, AD is the geometric surface area sampled, PD is the incident laser power density, t is the integration time, Ω is the solid angle of collection of the Raman scattered radiation, T is the monochromator transmission factor, Q is detector quantum efficiency, F is the transmission of the edge pass filter, and f is the surface area enhancement factor, the ratio of nanostructured sample area compared to a planar reference one. Here, f is determined as the ratio of the true surface area measured using electrochemical underpotential deposition of a

4

(1)

Eloc (r,ω) is the local electric field at point r and frequency ω. The origin of power of four in the formula is due to the fact that SPR effects both incident laser light and scattered light 18031



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Table 1. Experimental Signal, Background, and SEF Values for Thiophenol Monolayer pillar coating Ag Pt

PD (mW) 25 150

t (s)

signal intensitya (arb. unit)

background intensityb (arb. unit)

S/Nc

f

DS (nmol/cm2)

SEF

10 3600

3.8 (±0.7) × 10 4.2 (±0.2) × 103

1.6 (±0.2) × 10 1.0 (±0.3) × 104

5200 ± 300 13 ± 2

2.9 3.0

0.54 0.45

7.4 (±0.2) × 105 1

7

7

a Signal intensity taken at 997 cm−1. bBackground intensity taken as the root-mean-squared intensity at 800 cm−1. cS/N calculated as signal intensity (S) divided by noise intensity (N), where N is approximated as N = (S + B)1/2 using the background intensity (B).

Ag/PAN interface are also shown. The sum of these fits (red dashed trace) is overlaid on the raw spectral data for comparison. Of particular interest is the fit of the Ag (2) band at ∼1470 cm−1 to two components, one at 1472 cm−1 and one at 1466 cm−1. The former band is assigned to the native C60,25 but the lower frequency band, which decreases in frequency by 6 cm−1, is attributed to the anion radical of C60, resulting from charge transfer from Ag to the C60 film. Previous studies have shown the magnitude of this frequency decrease to be correlated with the extent of electron doping of C60,47−51 with a decrease of 6−7 cm−1 corresponding to the transfer of one electron per C60. Thus, the presence of this lower frequency component in the spectrum clearly indicates electron sharing/delocalization between the Ag SERS substrate and the C60 film. Assuming equal surface enhancement, the ratio of integrated intensities of the two Ag (2) bands is 4:1, suggesting that ∼80% of the C60 molecules are in the reduced state. Beyond electron transfer, no other spectral changes are observed, suggesting that no further reaction chemistry occurs. This is in contrast to the behavior of C60 with other low work function metals such as Al, whose deposition has been shown to induce reactions leading to a-C.52 Although in the present study, it is impossible to distinguish a-C originating at the Ag/ PAN interface from the one resulting from C60 degradation, the absence of any additional reaction product bands that are observed for Al-deposited substrates, is a good indication that Ag-to-C60 electron transfer does not induce further reaction chemistry.

metallic monolayer to the geometric area defined by the area of the working electrode from cyclic voltammetry. When considering the ratio of scattering intensities at Ag and Pt, the sample independent factors cancel, leaving only Ds, PD, t, and f to consider. Here, we use the values of Gui et al.35 for the surface number density (Ds) of thiophenol on our substrates. These values of 0.544 nmol/cm2 for Ag (111) and 0.45 nmol/ cm2 for Pt (111) are based on Auger electron spectroscopy. Table 1 summarizes collection parameters and signal and background intensities for thiophenol adsorbed on both Agand Pt-coated nanopillar arrays. The calculated SEF for the Agcoated pillars is shown with the SEF for Pt set to unity as reported by Taylor.22 Relative to Pt, the Ag-coated nanopillars exhibit a SEF on the order of 106, consistent with values observed from previously reported structured surfaces on which the structure was both ordered36−40 and disordered.41−44 The difference between experimental and simulation surface enhancement factors is usually attributed to charge transfer between the molecule and nanostructured surface. A charge transfer intermediate state effectively enhances the Raman signal through an electron-mediated resonance effect.45 Furthermore, a phase shift at the interface of the metal nanostructure and the dielectric may cause an additional enhancement in the electric field.46 In addition, the simulated enhancement factor reported here is underestimated as this is calculated over a two-dimensional cross section in the xy plane. As these nanostructured substrates may be of use for plasmonic enhancement of light absorption in organic solar cells, SERS was used to gain insight into the interfacial chemistry between the nanostructured Ag and C60, a commonly used electron acceptor material. Figure 8 shows the Raman spectrum from a 5 ML C60 film on Ag-coated nanopillar substrate (black trace). The results of spectral peak fitting of the C60 (green traces) and a-C (blue traces) modes at the buried

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: +1 (520) 6260936. Fax: +1 (520) 626-6219. Author Contributions §

A.A.K. and D.L.M. have equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is partially based upon work supported by the U.S. Air Force Office of Scientific Research under Award FA9550-10-1-0555 (BioPAINTS MURI). This research was supported as part of the Center for Interface Science: Solar Electric Materials (CISSEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DESC0001084 (D.L.M. and J.E.P.).

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Figure 8. Raman spectra of (a) C60 deposited on Ag-coated PAN nanostructures (black trace), (b) the sum of the fit results (dashed red trace, y axis offset included for clarity), (c) fit results attributed to α-C from Ag/PAN reaction at the interface (blue trace), and (d) bands fit for C60 and C60•− (green trace).

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Ag Interface of a Large Area Nanoarchitectured Ag

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