Fullerene Layers Coated

Article pubs.acs.org/JPCC

Investigating the Morphology of Polymer/Fullerene Layers Coated Using Orthogonal Solvents Christopher W. Rochester, Scott A. Mauger, and Adam J. Moulé* Department of Chemical Engineering and Material Science, University of CaliforniaDavis, Davis, California 95616, United States S Supporting Information *

ABSTRACT: P3HT/PCBM bilayer samples were fabricated by spin coating PCBM dissolved in CH2Cl2 onto P3HT films. Dissolution of the P3HT does not occur because CH2Cl2 is a nonsolvent for P3HT. We show using steady-state spectroscopy, neutron reflectometry, and current−voltage measurements that substantial mixing occurs between the P3HT and the PCBM during the PCBM deposition. Penetration of the PCBM into the P3HT layer is assisted by the swelling of the P3HT by the dichloromethane solvent. We conclude that the PCBM mixes with amorphous P3HT and does not disrupt the existing crystalline domains. A PCBM loading of 25−30 wt % into the P3HT layer was determined using fluorescence quenching measurements and neutron reflectometry. This amount of mixing explains why reported photovoltaic performances of these solution-processed bilayer structures are comparable to that of bulk heterojunctions.

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INTRODUCTION Polymer solar cells have the potential to be a low-cost alternative to conventional photovoltaic (PV) technologies. The low cost comes from the ability for these devices to be fabricated using simple, high-throughput solution deposition techniques. Polymer solar cells have reached power conversion efficiencies as high as 9.1%.1 These devices consist of bulkheterojunction (BHJ) active layers, where the donor and acceptor materials are mixed to form an interpenetrating network of nanosized domains that allow for efficient exciton harvesting.2−4 In contrast to BHJs, in bilayer structures, where the donor and acceptor material have a single planar junction, charge separation is limited due to the short exciton diffusion length for these materials.5,6 This leads to low short-circuit current densities (JSC) and therefore lower efficiencies. One advantage of bilayers is reduced dark current and improved charge transport, because once charge separation occurs the charges have an unobstructed path to their respective electrodes. Charge extraction is also more selective since only the electron-transporting material is in contact with the cathode and the hole-transporting material is in contact with the anode.7 There have been several reported studies on the fabrication of polymer/fullerene bilayers that include techniques such as thermal evaporation of C60 onto the polymer,8−10 floating the polymer layer onto a PCBM layer,11,12 stamping the two layers together using a PDMS stamp,13,14 and spin coating PCBM onto the polymer using orthogonal solvents.15−18 The spin coating technique is investigated in this study, because solution deposition techniques are more desirable due to their compatibility with high-throughput printing. PCBM is dissolved in CH2Cl2 and spin coated onto P3HT. We show here that a large amount of mixing occurs between the PCBM and © 2012 American Chemical Society

the P3HT when preparing the bilayers in this way. Substantial mixing has also been shown previously using neutron reflectometry.16 Polymer and fullerene mixing is not an uncommon occurrence. It has been observed by other studies that complete mixing between the two components will occur upon thermal annealing of the polymer/fullerene bilayer structure due to the inherent miscibility between P3HT and PCBM.11,12 Mixing is also known to occur while stamping polymer and fullerene layers together due to the heat and residual solvent present during the stamping process.19 For solution-processed bilayers fabricated using orthogonal solvents, we see that this mixing occurs spontaneously during deposition of the PCBM. In this study, bilayer films were examined using steady-state spectroscopy and neutron reflectometry (NR) to determine the extent of polymer/ fullerene mixing and observe the effects of different processing conditions.

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EXPERIMENTAL METHODS Bilayer samples were prepared on 25 × 25 mm glass substrates. The substrates were cleaned with chloroform and placed in sonic baths of acetone, mucasol (1 vol % in DI water), and deionized water (15 MΩ). The substrates were then rinsed and dried in a spin−rinser−dryer with deionized water and nitrogen gas. They were then placed in an UV−ozone generator for 30 min to remove any residual organic material. Once the cleaning process was complete, the substrates were placed in a nitrogenfilled glovebox. A P3HT solution was prepared by dissolving Received: December 21, 2011 Revised: February 28, 2012 Published: March 2, 2012 7287

dx.doi.org/10.1021/jp212341a | J. Phys. Chem. C 2012, 116, 7287−7292

The Journal of Physical Chemistry C

Article

measurements were taken in air before and after the samples were annealed on a hot plate set at 150 °C for 10 min. Neutron reflectivity measurements were performed on the Surface Profile Analysis Reflectometer (SPEAR) flight path at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory.21 A collimated neutron beam was directed at a sample at an incident angle of θ. The reflectivity R, defined as the ratio of the intensity of the specularly reflected neutron beam to that of the incident beam, was measured as a function of the momentum change perpendicular to the surface, Qz = (4n sin θ)/λ, also known as the momentum transfer vector. Here, λ is the wavelength of the neutrons. The measured reflectivity was fit to a slab model, in which the sample film was assumed to consist of a series of n parallel layers, where layer i has a thickness di and constant scattering length density (SLD) ρi, sandwiched between super- (air) and subphases (silicon) of infinite extent. Interlayer roughness, σi,i+1, which could include contributions from actual roughness between layers or from interlayer mixing, was accounted for by an error function SLD profile centered at the interface connecting the SLDs of the adjacent layers i and i + 1. The simplest model possible was used to represent the scattering length density (SLD) profiles of our samples. Additional layers were added when necessary to improve the fit. Model fitting to the measured data for log R vs Qz was carried out with the Levenberg−Marquardt nonlinear least-squares method using the MOTOFIT program22 in which the reflectivity profile is calculated using the Abeles matrix method.23 The scattering length densities of air and the silicon substrate were taken to be ρair = 0 and ρSi = 2.07 × 10−6 Å−2, respectively.22 Solar cell devices were fabricated onto ITO-coated glass substrates that were cleaned using the same process described above for the glass substrates. A 40 nm layer of PEDOT:PSS was deposited onto the substrates by spin coating at 2500 rpm. The BHJ active layer was spin coated from a 1:1 mix of P3HT:PCBM dissolved in 1,2-dichlorobenzene at 900 rpm to a thickness of 70 nm. For the bilayer devices, P3HT was cast from a 15 mg/mL solution in 1,2-dichlorobenzene at 1000 rpm to a thicknesses of 80 nm. A 5 mg/mL solution of PCBM dissolved in CH2Cl2 was spin coated onto the P3HT films at 4000 rpm. The sample was then placed on a hot plate set to 150 °C for 5 min. To complete the devices, a 10 nm Ca/80 nm Al electrode was deposited onto all samples in a thermal evaporator at a vacuum pressure of 5 × 10−6 mbar. Current− voltage (IV) measurements were performed under an AM1.5 spectrum at 100 mW/cm2 provided by a Radiant Source Technology solar simulator.

P3HT powder (Sigma-Aldrich) in chlorobenzene to a concentration of 15 mg/mL and stirring on a hot plate set to 60 °C for 2 h. A PCBM solution was made by dissolving PCBM powder in CH2Cl2 to a concentration of 5 mg/mL stirring at 60 °C for 1 h and was allowed to cool to room temperature before use. The P3HT solutions were spin coated onto the glass substrates at 1000 rpm. P3HT/PCBM bilayers were completed by depositing the PCBM solution directly on top of the P3HT films (4000 rpm). In order to quantify the amount of PCBM that mixes with the P3HT layer during deposition, the photoluminescence (PL) quenching of the P3HT in the P3HT/PCBM bilayer samples were measured and compared to BHJs with known PCBM concentrations. The PL quenching measurements were made with a Varian Cary Eclipse fluorescence spectrometer. The PL quenching efficiencies of the bilayers (QEBL) were calculated using the equation QEBL =

PLP − PLBL × 100% PLP

(1)

where PLP is the PL intensity of the pristine P3HT layer and PLBL is the PL intensity of the bilayer. In order to determine the relationship between PL quenching and PCBM loading into P3HT, a series of BHJs ranging from 20 to 70 wt % PCBM was made. Using a Perkin-Elmer UV−vis−NIR spectrophotometer, absorbance measurements of the BHJs were used to quantify the amount of P3HT in each sample. PL measurements of the BHJ samples were then used to determine how much of the P3HT PL is quenched by the PCBM. The PL intensity of the P3HT in each BHJ without the presence of PCBM (PLeqP) was determined using a calibration plot that compared the absorbance intensity of P3HT to its corresponding PL intensity, which was constructed by taking absorbance and PL spectra of pristine P3HT films of different thicknesses. The quenching efficiencies of the BHJs (QEBHJ) were then computed using eq 2. QEBHJ =

PLeqP − PLBHJ PLeqP

× 100% (2)

Here, PLeqP is the PL intensity for the equivalent amount of P3HT in the BHJ sample without PCBM present and PLBHJ is the measured PL intensity of the BHJ sample. This method allowed us to construct a calibration curve that shows the relationship between PCBM loading and the PL quenching efficiency. For the neutron reflectivity (NR) measurements, PEDOT:PSS was coated onto a silicon wafer substrate by spin coating at 3200 rpm for 60 s. It was then heated at 115 °C for 5 min and immediately transported to a nitrogen-filled glovebox. A solution of P3HT dissolved in 1,2-dichlorobenzene at 15 mg/ mL was spin coated onto PEDOT:PSS at 1000 rpm for 60 s and ramped up to 2000 rpm for 35 s to dry the film. The PCBM solution was then spin coated onto the P3HT at 4000 rpm for 15 s. Finally, 20 nm of aluminum was deposited onto the sample by thermal evaporation, operating at a pressure of 5 × 10−6 mbar. A BHJ sample of the same layered structure was also prepared for comparison. Thickness uniformity was verified using a dektak surface profiler. The thickness variation over the entire sample area was less than ±2 nm. Since the domain sizes that are formed in a P3HT:PCBM BHJ are less than 10 nm,20 the observed scattering length density is representative of the average composition of the layer. NR

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RESULTS AND DISCUSSION The solution-processed P3HT/PCBM bilayers were first analyzed using steady-state spectroscopy measurements. Figure 1 shows the absorbance and photoluminescence (PL) spectra of an 80 nm pristine P3HT layer and a P3HT/PCBM bilayer. After the bilayer sample was measured, it was then washed with CH2Cl2 to remove the PCBM and measured again. From the PL spectra, it is seen that after depositing the PCBM onto the P3HT layer, over 95% of the P3HT fluorescence is quenched. Since the exciton diffusion length in P3HT is known to be 5− 10 nm,5,6 over 95% quenching for a 80 nm P3HT layer is not expected for a bilayer structure. This indicates that a substantial amount of PCBM is diffusing into the P3HT layer during the PCBM solution deposition (Figure 1c), allowing for very efficient exciton quenching. This fluorescence quenching result 7288



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is a poor solvent for P3HT. The lower absorbance intensity of the bilayer compared to the absorbance after washing is due to the lower reflectance of the bilayer. To verify that the return of the P3HT fluorescence after washing is a result of the PCBM being washed out of the P3HT film and not just from the surface, a BHJ sample consisting of P3HT mixed with 30 wt % PCBM was prepared. The absorbance and PL spectra for this sample before and after washing with CH2Cl2 are displayed in Figure 2. We observed

Figure 2. (a) UV−vis and (b) photoluminescence spectrum of a P3HT film containing 30 wt % PCBM before and after washing with CH2Cl2.

that after washing the film the absorbance peak at 340 nm disappears and the PL intensity increases 20-fold, confirming that all of the PCBM is washed away even when it is cast from a mixed solution. Since CH2Cl2 can easily swell the P3HT film to redissolve and remove the PCBM, a PCBM solution in CH2Cl2 can just as easily deposit PCBM into the polymer layer. One important detail that is observed in the absorption spectrum of Figure 1a is that the crystallinity of the P3HT is maintained even though PCBM goes into the polymer film, as shown by the presence of the absorbance peak at 610 nm. It has been shown that when casting from a mixed solution of P3HT and PCBM the P3HT crystallinity in the film is decreased by the presence of the PCBM molecules, which hinder the π−π stacking of the polymer chains.24 Since the crystallinity of the P3HT layer is maintained after casting the PCBM, this means that the fullerenes must reside within the free volume of the amorphous regions between the crystalline domains. This conclusion is consistent with previous studies that showed that PCBM mixes with the amorphous P3HT after thermal

Figure 1. (a) UV−vis and (b) photoluminescence spectrum of a P3HT film (solid line), a P3HT/PCBM bilayer (dashed), and a P3HT/PCBM bilayer washed with CH 2Cl2 (dot−dash). (c) Illustration showing the resultant sample after solution deposition of PCBM onto a P3HT layer. Rather than two distinct nonmixing layers (top right) a substantial amount of PCBM diffuses into the P3HT layer (bottom right).

was also observed when using cold toluene (5 °C) as an orthogonal solvent. After washing the sample with CH2Cl2, it is seen in Figure 1a that the PCBM absorption peak at 340 nm disappears and the P3HT fluorescence almost completely returns. The disappearance of the PCBM absorption peak and the return of the P3HT fluorescence intensity shows that the PCBM can be removed from the film using CH2Cl2. The CH2Cl2 readily penetrates into the polymer layer even though it 7289



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annealing of P3HT/PCBM bilayers as the PCBM diffuses into the P3HT.11,12 In order to quantify the amount of PCBM that goes into the P3HT layer during deposition, the PL quenching of the P3HT/ PCBM bilayer samples was compared to BHJs with known PCBM concentrations. Using eq 2, a plot of quenching efficiency vs PCBM concentration was constructed. Using eq 1 the quenching efficiencies for the bilayers were measured to be 96 ± 0.9%. Varying the P3HT thicknesses in the bilayers showed no change in the quenching efficiency. Since there is no variation of quenching efficiency with P3HT thickness, this means that the PCBM loading is uniform and penetrates all the way through the film. We found that a 96 ± 0.9% quenching efficiency corresponds to a PCBM concentration of 30 ± 8 wt %, which is consistent with NR measurements shown in Figure 4. This is substantial mixing and is responsible for the high PL quenching and good PV performances that have been observed by other researchers.15,16 After measuring the degree of mixing that occurs during PCBM deposition, the effects of different processing conditions were examined. Figure 3 shows the PL spectrum of three

Figure 3. PL spectrum for a bilayer with an as-cast P3HT layer, a bilayer with the P3HT layer annealed prior to PCBM deposition, and a bilayer with an as-cast P3HT layer and the PCBM layer cast from a dilute (4 mg/mL) solution.

bilayers fabricated using different processing steps. The first sample is a bilayer in which an 8 mg/mL PCBM/CH2Cl2 solution was cast onto an as-cast P3HT layer. In the second sample an 8 mg/mL PCBM/CH2Cl2 solution was cast onto an annealed (150 °C, 10 min) P3HT layer. The third sample consists of PCBM cast from a dilute solution (4 mg/mL) onto an as-cast P3HT film. We observe that changing the concentration of the PCBM solution has little effect on the PL intensity, and thereby PCBM loading, of the bilayer samples. However, annealing the P3HT layer prior to PCBM deposition causes the PL intensity to be nearly twice that of the nonannealed P3HT sample, showing that the amount of PCBM that mixes into the P3HT is dependent only upon the packing of the P3HT chains in the film. Annealing the P3HT film causes ordering of the polymer chains into densely packed crystalline domains that hinder the intrusion of PCBM molecules. Since the PCBM can only diffuse through the free volume of the amorphous regions in P3HT, the increased crystallinity of the P3HT film will reduce the space available for PCBM to reside. Figure 4 shows the NR plot, scattering length density (SLD) profile, and PCBM volume percent profile of the P3HT/PCBM

Figure 4. Neutron reflectivity measurements for P3HT/PCBM bilayers on PEDOT:PSS. (a) Reflectivity versus Qz of bilayers before (blue, square) and after (green, circle) heating at 150 °C for 10 min. (b) Scattering length density profiles versus thickness. (c) Volume % PCBM versus normalized thickness calculated from the SLD profile. Profile is restricted to the P3HT and PCBM layers.

bilayers before and after annealing at 150 °C for 10 min. The sample structure is Si/PEDOT:PSS/P3HT/PCBM/Al. The SLD profile for the as-cast sample shows a 20 nm layer of PCBM on top of a 110 nm P3HT:PCBM mixed layer. To calculate the volume % PCBM, the SLDs used for pure P3HT and PCBM were 0.786 × 10 −6 and 3.70 × 10 −6 Å −2 respectively.25 This PCBM SLD value corresponds to a mass density of 1.28 g/cm2. This mass density for PCBM is consistent with a measurement performed in a previous study using neutron reflectivity.26 Figure 4c displays the PCBM volume percent profile in the active layer (excluding the PEDOT:PSS and the Al electrode) and shows that spontaneous 7290



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mixing occurs in both layers in the as-cast sample. The 55 vol % of PCBM in the top layer is due to slight dissolution of the underlying P3HT layer during the PCBM deposition, at the same time the PCBM solution is swelling into the P3HT layer causing the layer to fill with 23 vol % PCBM. This 23 vol % is equivalent to 26 wt % given the densities of the individual components. After annealing the sample, slight vertical segregation of the PCBM occurs. The PCBM loading in the P3HT layer drops to 21 vol% (25 wt %), and the top layer raises to 81 vol%. This weight fraction of PCBM in the P3HT layer is close to what was measured using PL quenching measurements and shows that a large amount of mixing occurs between the P3HT and the PCBM. The PCBM-rich top layer is desirable as it serves as an electron-selective transporting layer in a photovoltaic device. Inclusion of such a layer should reduce dark current and therefore enhance VOC.7 For comparison, NR was also performed on a BHJ sample with a 1:1 P3HT:PCBM weight ratio with the structure of Si/ PEDOT:PSS/BHJ/Al before and after heating at 150 °C for 10 min (Figure 5). Before heating the sample, both top and bottom interfaces are PCBM rich, which is in agreement with a previous neutron reflectivity study.26 A thin layer of P3HT is often observed on top of the BHJ, but it cannot be seen in Figure 5 because of the roughness of the BHJ/Al interface.27 After heating, the PCBM further segregates to the interfaces. This vertical concentration profile is similar to the bilayer except for the PCBM-rich region at the PEDOT:PSS interface. For this study, identical coating and annealing methods were used for both the bilayer and the BHJ samples, which may not be the optimal conditions for both. The high PCBM concentration at the PEDOT:PSS interface is not desirable for a photovoltaic device as it may hinder hole transport across the anode interface. This is why the bilayer fabrication method may be more beneficial due to better control of the vertical profile. The facile penetration of PCBM into P3HT explains the good photovoltaic performance of the bilayer solar cells that have been previously reported.15,16 If the bilayers consisted of nonmixing polymer and fullerene layers, then only 5−10 nm of the P3HT layer adjacent to the P3HT/PCBM interface would contribute to photocurrent generation due to the limited exciton diffusion length. This would result in a low current density for the device. The current−voltage (IV) plot of a bilayer device with an 80 nm P3HT film and a BHJ with a 1:1 P3HT:PCBM weight ratio is displayed in Figure 6. Here it is shown that the bilayer device is comparable to the BHJ. The bilayer device has a short-circuit current density (JSC) of 10.0 mA/cm2, which is higher than the BHJ JSC of 7.7 mA/cm2. The difference in JSC is mainly due to the greater thickness of the bilayer device, causing it to absorb more light. The VOCs are identical at 0.66 V for the two devices, and the bilayer has a slightly higher fill factor (FF = 0.59) than the BHJ (FF = 0.56). The higher FF may be attributed to the more ideal PCBM vertical concentration profile achieved by the bilayer. The power conversion efficiencies of the BHJ and bilayer were 2.9% and 4.0%, respectively. The similar performances of the two devices is attributed to their similar active layer morphology due to the mixing that occurs between the P3HT and the PCBM. In conclusion, we showed that spontaneous mixing occurs between P3HT and PCBM when casting PCBM onto a P3HT film using an orthogonal solvent. This mixing occurs because of P3HT swelling in the presence of poor solvents. Using

Figure 5. Neutron reflectivity measurements for BHJ films on PEDOT:PSS. (a) Reflectivity versus Qz of BHJ before (blue, square) and after (red, circle) heating at 150 °C for 10 min. (b) Scattering length density profiles versus thickness. (c) Volume % PCBM versus normalized thickness calculated from the SLD profile. Profile is restricted to the BHJ active layer.

photoluminescence quenching and neutron reflectivity measurements, we were able to determine the PCBM loading into the P3HT layer to be 25−30 wt %. We conclude that the PCBM resides in the amorphous portions of the P3HT film and do not break apart the existing P3HT crystal domains. We were able to show that the amount of mixing depends only on the initial crystallinity of the P3HT film. The mixing was found to be uniform throughout the layer as observed from the thickness dependent PL measurements and neutron reflectivity experiments. The mixing of the PCBM into the P3HT and the presence of the PCBM top layer make these active layer structures ideal for efficient photovoltaic devices. The bilayer fabrication method offers better control of the vertical 7291



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(10) Stevens, D. M.; Qin, Y.; Hillmyer, M. A.; Frisbie, C. D. J. Phys. Chem. C 2009, 113, 11408−11415. (11) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Adv. Energy Mater. 2011, 1, 82−89. (12) Collins, B. A.; Gann, E.; Guignard, L.; He, X.; McNeill, C. R.; Ade, H. J. Phys. Chem. Lett. 2010, 1, 3160−3166. (13) Huang, J. H.; Ho, Z. Y.; Kuo, T. H.; Kekuda, D.; Chu, C. W.; Ho, K. C. J. Mater. Chem. 2009, 19, 4077−4080. (14) Kim, J. B.; Lee, S.; Toney, M. F.; Chen, Z. H.; Facchetti, A.; Kim, Y. S.; Loo, Y. L. Chem. Mater. 2010, 22, 4931−4938. (15) Ayzner, A. L.; Tassone, C. J.; Tolbert, S. H.; Schwartz, B. J. J. Phys. Chem. C 2009, 113, 20050−20060. (16) Lee, K. H.; Schwenn, P. E.; Smith, A. R. G.; Cavaye, H.; Shaw, P. E.; James, M.; Krueger, K. B.; Gentle, I. R.; Meredith, P.; Burn, P. L. Adv. Mater. 2010, 23, 766−770. (17) Wang, D. H.; Lee, H. K.; Choi, D. G.; Park, J. H.; Park, O. O. Appl. Phys. Lett. 2009, 95, 043505. (18) Kaur, M.; Gopal, A.; Davis, R.; Heflin, J. Sol. Energy Mater. Sol. Cells 2009, 93, 1779−1784. (19) Wang, D. H.; Choi, D. G.; Lee, K. J.; Im, S. H.; Park, O. O.; Park, J. H. Org. Electron. 2010, 11, 1376−1380. (20) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Adv. Funct. Mater. 2005, 15, 1193−1196. (21) Dubey, M.; Jablin, M. S.; Wang, P.; Mocko, M.; Majewski, J. Eur. Phys. J. Plus 2011, 126. (22) Nelson, A. J. Appl. Crystallogr. 2006, 39, 273−276. (23) Heavens, O. Optical Properties of Thin Solid Film; Dover: New York, 1991. (24) Swinnen, A.; Haeldermans, I.; Vanlaeke, P.; D’Haen, J.; Poortmans, J.; D’Olieslaeger, M.; Manca, J. V. Eur. Phys. J.: Appl. Phys. 2006, 36, 251−256. (25) Huang, D. M.; Mauger, S. A.; Friedrich, S.; George, S. J.; Dumitriu-LaGrange, D.; Yoon, S.; Moulé, A. J. Adv. Funct. Mater. 2011, 21, 1657−1665. (26) Kiel, J. W.; Kirby, B. J.; Majkrzak, C. F.; Maranville, B. B.; Mackay, M. E. Soft Matter 2010, 6, 641−646. (27) Germack, D. S.; Chan, C. K.; Kline, R. J.; Fischer, D. A.; Gundlach, D. J.; Toney, M. F.; Richter, L. J.; DeLongchamp, D. M. Macromolecules 2010, 43, 3828−3826.

Figure 6. IV plots of a 1:1 BHJ device, a bilayer device with an 80 nm thick P3HT layer, and a bilayer device with a 50 nm thick P3HT layer.

concentration profile over conventional BHJ fabrication techniques.

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ASSOCIATED CONTENT

S Supporting Information *

PL quenching data and calibration curves, effect of CH2Cl2 on the absorbance spectrum of P3HT, and weight percent profile of PCBM on the bilayer sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the Department of Energy under Award Number DOE FG36-08GO18018. This work benefited from the help of Peng Wang with the use of the Lujan Neutron Scattering Center at LANSCE funded by the DOE Office of Basic Energy Sciences and Los Alamos National Laboratory under DOE Contract DE-AC52-06NA25396.

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REFERENCES

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Fullerene Layers Coated

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