Hot Hole Transfer Increasing Polaron Yields in Hybrid Conjugated

Letter pubs.acs.org/JPCL

Hot Hole Transfer Increasing Polaron Yields in Hybrid Conjugated Polymer/PbS Blends Elisabeth Strein, Dane W. deQuilettes, Stephen T. Hsieh, Adam E. Colbert, and David S. Ginger* Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States S Supporting Information *

ABSTRACT: We use quasi-steady-state photoinduced absorption (PIA) to study charge generation in blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) with PbS nanocrystal quantum dots as a function of excitation energy. We find that, per photon absorbed, the yield of photogenerated holes present on the conjugated polymer increases with pump energy, even at wavelengths where only the quantum dots absorb. We interpret this result as direct evidence for transfer of hot holes in these conjugated polymer/quantum dot blends. These results help understand the operation of hybrid organic/inorganic photovoltaics. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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information on the role of hot charge carriers on the way to photocurrent generation in hybrid devices. Here, we look for evidence of hot hole transfer from photoexcited quantum dots to conjugated polymers by measuring the yield of long-lived charge carriers in polymer/ quantum dot hybrids as a function of excitation wavelength. We chose a blend of poly-3-hexylthiophene (P3HT) with PbS quantum dots as our model system because the comparatively narrow gap of P3HT allows us to spectrally isolate the photoexcitation of the quantum dots from the excitation of the polymer over a wider range than would be possible with previous polymers that we have studied.28 Although blends of larger PbS dots with P3HT show little evidence of photoinduced CT,29 smaller-sized (larger-gap) PbS dots, when treated with the appropriate ligand exchange protocols (see the Supporting Information (SI)), do produce measurable yields of long-lived charge under photoexcitation, making them a useful system for our study. We prepared samples for photoinduced absorption (PIA) spectroscopy by spin coating blend solutions of P3HT and PbS quantum dots in a nitrogen glovebox and treating them with 10 mM 3-mercaptopropionic acid (MPA) solution (see SI section 1). We selected MPA to passivate PbS because of its reported high carrier mobility lifetime product.30,31 Furthermore, we selected a sample that exhibited minimal photoluminescence (PL) because the residual PL signal from smaller-sized PbS quantum dots can interfere with the P3HT polaron signal (see the SI, section 1). We used PIA to probe the long-lived excited states that were formed following the photoexcitation of P3HT/PbS blends.

nderstanding the factors that regulate the efficiency of charge generation at donor/acceptor interfaces in excitonic solar cells is critical to improving the performance of a number of emerging solar energy technologies ranging from quantum dots1−4 and organic photovoltaics5,6 to hybrid solar cells including both polymer/quantum dot blends7−11 and emerging hybrid organic/perovskite devices.12,13 Especially in the case of cells relying on condensed-phase donor/acceptor heterojunctions, the details of charge generation remain a topic of active investigation.14−17 Nevertheless, while the overall design rules are not understood, current thinking is that factors such as wave function delocalization,18−20 charge transfer (CT) from nonthermalized “hot” states,21−23 the total driving force for CT,16,24 coupling to higher-lying CT states,17,25 interfacial dipoles,17,26 and local electric fields14 can all play a role. Recent studies have shown that in various polymer/fullerene blends, free carriers can be generated very quickly (sub-100 fs) and can benefit from excess energy above the band gap.25 Similarly, in all-inorganic systems, ultra-fast hot electron injection from PbS and PbSe quantum dots to TiO2 nanocrystals has been observed to be in competition with electron relaxation.23,27 Compared to polymer/fullerene blends, much less is known about the mechanisms for charge generation in the hybrid polymer/quantum dot systems. We have previously reported that the internal quantum efficiency (IQE) increases proportionally to excitation energy in polymer/PbS hybrid solar cells.8 More recently, we reported data that could be interpreted as evidence for transfer of hot holes from photoexcited quantum dots to a conjugated polymer.28 Although these results are suggestive, it has so far been difficult to separate different rates of quantum-dot-to-polymer hole transfer and polymer-toquantum-dot electron transfer from the signature of hot hole transfer in charge generation. In this Letter, we probe the role of hot hole transfer in polaron formation, giving important © 2013 American Chemical Society

Received: November 5, 2013 Accepted: December 16, 2013 Published: December 16, 2013 208

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PIA is a pump−probe technique measuring changes in optical transmission due to the formation of long-lived excited states such as polarons with lifetimes on the micro- to millisecond time scale.29,32−34 To probe the polaron population as a function of pump photon energy, we used five LEDs as pump sources, 447 (2.77 eV), 660 (1.88 eV), 740 (1.68 eV), 850 (1.46 eV), and 950 nm (1.31 eV). (Experimental details are included in the SI section 2.) Figure 1 shows the UV−vis spectra of the P3HT polymer film, the PbS quantum dot film, and the P3HT/PbS film. Figure

with a shoulder at around 1.25 eV is characteristic of P3HT polarons29,32 and matches the polaron spectrum of a control P3HT/fullerene blend (see Figure SI-2.4, SI). Figure 2 also shows that regardless of pump excitation energy, the shapes of the PIA spectra are identical, indicating that we are probing the same polaron species. Because the PIA signal magnitude is dependent upon the absorbed excitation flux, it is important to compare dT/T data between different pump energies at the same absorbed photon flux. For each pump wavelength, we systematically varied the excitation flux and determined the corresponding polaron yield from the PIA signal at 1050 nm. Figure 3 is the key figure of

Figure 1. UV−vis absorption spectra of a P3HT film (gray dashed line), a thin film of PbS quantum dots (dotted line), and a P3HT/PbS thin film blend (solid black line). The colored peaks are the (normalized) spectra of the various pump LEDs. From left to right, the pumps are 447 nm with a 450 nm band-pass filter, 660 nm, 740 nm with a 740 nm band-pass filter, 850 nm, and finally 950 nm. We note the P3HT absorption does not overlap significantly with the 740, 850, or 950 nm pumps.

Figure 3. Intensity dependence of the PIA polaron signal at 1050 nm (1.18 eV) for different pump energies as a function of absorbed photon flux. The straight lines are fits to the data, as explained in the text. The fit equations are as follows: dT/T447 = 5.30 × 10−14(Φ)0.59, dT/T660 = 4.48 × 10−14(Φ)0.59, dT/T740 = 2.94 × 10−14(Φ)0.59, dT/ T850 = 2.16 × 10−14(Φ)0.59, and dT/T950 = 1.72 × 10−14(Φ)0.59. For clarity, we display only one data set for each pump excitation wavelength; however, the fits are best fits to the entire set of repeated measurements with each pump LED. For additional fitting details and the full data set, see the Supporting Information.

1 also shows the normalized spectra of the various pump sources used in these experiments. Notably, the 740, 850, and 950 nm LEDs have virtually no overlap with the P3HT absorption spectrum and thus selectively excite the PbS dots. The fwhm of the absorption peak of the quantum dots is ∼140 meV. As a result, there will always be some dispersion in valence band offset between the quantum dots and the polymer. However, this dispersion is small compared to the range of wavelengths over which we varied the pump energy (1.46 eV). Figure 2 shows normalized photoinduced absorption spectra of the P3HT/PbS film taken in the NIR region with each of the five different pumps. The broad induced absorption feature

this Letter and plots the fractional change in probe beam transmission at 1050 nm (1.18 eV) as a function of absorbed photon flux (incident photon flux corrected by sample absorption), which we determined in the following manner Φabs =

∫ ΦP(λ)[1 − 10−OD(λ)] dλ

where Φabs is the number of photons absorbed per second, ΦP(λ) is the photon flux coming from the LED at a given wavelength, and OD(λ) is the wavelength-dependent optical density as measured from the sample’s absorbance spectrum. In other words, Figure 3 shows the intensity dependence of the polaron signal for all five studied pump wavelengths as a function of the number of photons absorbed by the sample each second. Per absorbed photon, the longest wavelength excitation is the least effective at generating long-lived polarons, and the shortest wavelength excitation is the most effective. The other wavelengths fall in between, with the polaron signal at a given absorbed flux increasing monotonically as the energy of the pump photons increases. In order to analyze the data more quantitatively, we performed a global power law fit of PIA signal versus absorbed photon flux of the form dT = A(Φabs)b T where A and b serve as fit parameters and b is constrained to fall between 0.5 and 1. We chose this fit function based on the expected dependence of the quasi-CW PIA signal on the

Figure 2. X-channel (in phase) PIA spectra for a P3HT/PbS blend excited with five different pump energies. The broad photoinduced absorption peak from 950 to 1150 nm is the well-identified polaron peak of P3HT. The spectral shape of the PIA feature is independent of excitation energy. 209

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polaron generation rate (absorbed photon flux).35−37 For purely first-order recombination, dT/T should be linear in Φabs photon flux (b = 1.0).36 For purely second-order (bimolecular recombination), dT/T should scale as a power law in Φabs with b = 0.5.37 In a dispersive system, the behavior is more complicated.35 A global fit over multiple data sets (see the SI) yields b = 0.59, describes the data in Figure 3 well, and is also in good agreement with the intensity dependence previously reported for a different conjugated polymer/PbS blend.28 The power law exponent of 0.59 is close to the value of 0.5 expected for a second-order process and suggests that bimolecular carrier recombination is the primary decay pathway in these samples under PIA conditions. Consistent with this analysis, we assume that the measured polarons are primarily decaying via bimolecular processes, in which case the polaron generation rate, g, will be proportional to the square of the observed dT/T signal.36,37 Figure 4 plots

energy, a result that we interpret as being due to the transfer of nonthermalized holes from the photoexcited quantum dots to the polymer host. This finding might help explain the wavelength-dependent internal quantum efficiencies that have been previously reported for polymer/PbS blend solar cells8 and is consistent with hole transfer occurring faster than the typical intraband carrier relaxation times in surface-treated PbS dots in these blends. In addition to explaining the performance characteristics of hybrid bulk heterojunction solar cells, we expect that these results might further be useful in designing organic/inorganic heterojunctions for use in exciton fission and carrier multiplication schemes where there may be a need to optimize relative CT and carrier cooling rates.

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ASSOCIATED CONTENT

S Supporting Information *

PbS quantum dot synthesis, sample fabrication, and sample photoluminescence are included in the first section. Section 2 has experimental details of the PIA data in Figure 3 and has figures that show (1) repeated measurements for the figure and their associated fit equations, (2) the dependence of PIA signal on the pump chopping frequency, and (3) comparison of the PbS/P3HT PIA spectrum to a PCBM/P3HT spectrum. Section 3 provides details of a verification experiment that uses a chopper and a range of ND filters to ensure that the data in Figure 3 are independent of electronic artifacts. 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].

2

Figure 4. Polaron generation rate (proportional to (dT/T) ) plotted versus the pump energy. The symbols are average values of two sets of data, and the error bars are the standard deviations. The dashed fit line from 1.7 to 3 eV indicates the regime where both the polymer and quantum dots absorb. The solid fit line below 1.7 eV indicates where only the quantum dots are contributing to the film absorption.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was primarily supported by DOE BES DE-FG0207ER46467.

the normalized generation rate g (the square of the prefactor A from the fits in Figure 3) as a function of pump energy. We see that the generation rate increases monotonically with pump energy, as expected based on the visible trends from Figure 3. Because the three lowest-energy pump wavelengths are not absorbed by the P3HT, we conclude that hole transfer from the quantum dots to the polymer is strongly dependent on the energy of the absorbed photon. This result indicates that hole transfer must be taking place, at least at some of the P3HT/PbS interfaces, on time scales faster than the carrier cooling times of ∼tens of picoseconds that have been measured for PbS quantum dots.38,39 Our interpretation of Figure 3 in terms of hot hole transfer is thus consistent with the subpicosecond hole transfer times that Siebbeles and co-workers have previously measured in PbS/P3HT blends (albeit with different surface ligands).40 Moving to the higher pump photon energies in Figure 4, the data suggest a further increase in the relative generation rate when both the quantum dots and the polymer are photoexcited. Although we can only speculate with the current data set, we suggest that this trend could result from a combination of hot hole transfer and a more favorable rate of polymer-toquantum dot electron transfer compared with quantum-dot-topolymer hole transfer.28 To conclude, in a model P3HT/PbS blend, we observe increasing polymer polaron yields with increasing pump photon

REFERENCES

(1) Kim, J. Y.; Voznyy, O.; Zhitomirsky, D.; Sargent, E. H. 25th Anniversary Article: Colloidal Quantum Dot Materials and Devices: A Quarter-Century of Advances. Adv. Mater. 2013, 25, 4986−5010. (2) ten Cate, S.; Liu, Y.; Schins, J. M.; Law, M.; Siebbeles, L. D. A. Phonons Do Not Assist Carrier Multiplication in PbSe Quantum Dot Solids. J. Phys. Chem. Lett. 2013, 4, 3257−3262. (3) Choi, J. J.; Lim, Y. F.; Santiago-Berrios, M. B.; Oh, M.; Hyun, B. R.; Sung, L. F.; Bartnik, A. C.; Goedhart, A.; Malliaras, G. G.; Abruna, H. D.; et al. PbSe Nanocrystal Excitonic Solar Cells. Nano Lett. 2009, 9, 3749−3755. (4) Choi, J. J.; Wenger, W. N.; Hoffman, R. S.; Lim, Y. F.; Luria, J.; Jasieniak, J.; Marohn, J. A.; Hanrath, T. Solution-Processed Nanocrystal Quantum Dot Tandem Solar Cells. Adv. Mater. 2011, 23, 3144. (5) Darling, S. B.; You, F. The Case for Organic Photovoltaics. RSC Adv. 2013, 3, 17633−17648. (6) Vandewal, K.; Himmelberger, S.; Salleo, A. Structural Factors That Affect the Performance of Organic Bulk Heterojunction Solar Cells. Macromolecules 2013, 46, 6379−6387. (7) Piliego, C.; Manca, M.; Kroon, R.; Yarema, M.; Szendrei, K.; Andersson, M. R.; Heiss, W.; Loi, M. A. Charge Separation Dynamics in a Narrow Band Gap Polymer−PbS Nanocrystal Blend for Efficient Hybrid Solar Cells. J. Mater. Chem. 2012, 22, 24411−24416. (8) Noone, K. M.; Strein, E.; Anderson, N. C.; Wu, P. T.; Jenekhe, S. A.; Ginger, D. S. Broadband Absorbing Bulk Heterojunction Photovoltaics Using Low-Bandgap Solution-Processed Quantum Dots. Nano Lett. 2010, 10, 2635−2639.

210

dx.doi.org/10.1021/jz402383x | J. Phys. Chem. Lett. 2014, 5, 208−211

The Journal of Physical Chemistry Letters

Letter

(28) Colbert, A. E.; Janke, E. M.; Hsieh, S. T.; Subramaniyan, S.; Schlenker, C. W.; Jenekhe, S. A.; Ginger, D. S. Hole Transfer from Low Band Gap Quantum Dots to Conjugated Polymers in Organic/ Inorganic Hybrid Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 280−284. (29) Noone, K. M.; Anderson, N. C.; Horwitz, N. E.; Munro, A. M.; Kulkarni, A. P.; Ginger, D. S. Absence of Photoinduced Charge Transfer in Blends of PbSe Quantum Dots and Conjugated Polymers. ACS Nano 2009, 3, 1345−1352. (30) Jeong, K. S.; Tang, J.; Liu, H.; Kim, J.; Schaefer, A. W.; Kemp, K.; Levina, L.; Wang, X.; Hoogland, S.; Debnath, R.; et al. Enhanced Mobility-Lifetime Products in PbS Colloidal Quantum Dot Photovoltaics. ACS Nano 2012, 6, 89−99. (31) Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; et al. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577−582. (32) Noone, K. M.; Subramaniyan, S.; Zhang, Q.; Cao, G.; Jenekhe, S. A.; Ginger, D. S. Photoinduced Charge Transfer and Polaron Dynamics in Polymer and Hybrid Photovoltaic Thin Films: Organic vs Inorganic Acceptors. J. Phys. Chem. C 2011, 115, 24403−24410. (33) Ginger, D. S.; Greenham, N. C. Charge Separation in Conjugated-Polymer/Nanocrystal Blends. Synth. Met. 1999, 101, 425−428. (34) Ginger, D. S.; Greenham, N. C. Photoinduced Electron Transfer from Conjugated Polymers to CdSe Nanocrystals. Phys. Rev. B 1999, 59, 10622−10629. (35) Wohlgenannt, M.; Ehrenfreund, E.; Vardeny, Z. V. Spectroscopy of Long-Lived Photoexcitations in π-Conjugated Systems. In Photophysics of Molecular Materials: From Single Molecules to Single Crystals; Lanzani, G., Ed.; Wiley-VCH: Weinheim, Germany, 2006; pp 183− 259. (36) Botta, C.; Luzzati, S.; Tubino, R.; Bradley, D. D. C.; Friend, R. H. Photoinduced Absorption of Polymer-Solutions. Phys. Rev. B 1993, 48, 14809−14817. (37) Dellepiane, G.; Cuniberti, C.; Comoretto, D.; Musso, G. F.; Figari, G.; Piaggi, A.; Borghesi, A. Long-Lived Photoexcited States in Symmetrical Polydicarbazolyldiacetylene. Phys. Rev. B 1993, 48, 7850− 7856. (38) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots. Nano Lett. 2005, 5, 865−871. (39) Stewart, J. T.; Padilha, L. A.; Qazilbash, M. M.; Pietryga, J. M.; Midgett, A. G.; Luther, J. M.; Beard, M. C.; Nozik, A. J.; Klimov, V. I. Comparison of Carrier Multiplication Yields in PbS and PbSe Nanocrystals: The Role of Competing Energy-Loss Processes. Nano Lett. 2012, 12, 622−628. (40) ten Cate, S.; Schins, J. M.; Siebbeles, L. D. A. Origin of Low Sensitizing Efficiency of Quantum Dots in Organic Solar Cells. ACS Nano 2012, 6, 8983−8988.

(9) Strein, E.; Colbert, A.; Subramaniyan, S.; Nagaoka, H.; Schlenker, C. W.; Janke, E.; Jenekhe, S. A.; Ginger, D. S. Charge Generation and Energy Transfer in Hybrid Polymer/Infrared Quantum Dot Solar Cells. Energy Environ. Sci. 2013, 6, 769−775. (10) Greaney, M. J.; Das, S.; Webber, D. H.; Bradforth, S. E.; Brutchey, R. L. Improving Open Circuit Potential in Hybrid P3HT:CdSe Bulk Heterojunction Solar Cells via Colloidal tertButylthiol Ligand Exchange. ACS Nano 2012, 6, 4222−4230. (11) Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.; Rumbles, G. Photovoltaic Devices with a Low Band Gap Polymer and CdSe Nanostructures Exceeding 3% Efficiency. Nano Lett. 2010, 10, 239− 242. (12) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (13) Liu, M. Z.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395. (14) Nayak, P. K.; Narasimhan, K. L.; Cahen, D. Separating Charges at Organic Interfaces: Effects of Disorder, Hot States, and Electric Field. J. Phys. Chem. Lett. 2013, 4, 1707−1717. (15) Dimitrov, S. D.; Durrant, J. R. Materials Design Considerations for Charge Generation in Organic Solar Cells. Chem. Mater. 2013. (16) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736−6767. (17) Brédas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: The Challenges. Acc. Chem. Res. 2009, 42, 1691−1699. (18) Rao, A.; Chow, P. C. Y.; Gelinas, S.; Schlenker, C. W.; Li, C.-Z.; Yip, H.-L.; Jen, A. K. Y.; Ginger, D. S.; Friend, R. H. The Role of Spin in the Kinetic Control of Recombination in Organic Photovoltaics. Nature 2013, 500, 435−439. (19) Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.; Cerullo, G.; Lanzani, G. Hot Exciton Dissociation in Polymer Solar Cells. Nat. Mater. 2013, 12, 29−33. (20) Bansal, N.; Reynolds, L. X.; MacLachlan, A.; Lutz, T.; Ashraf, R. S.; Zhang, W. M.; Nielsen, C. B.; McCulloch, I.; Rebois, D. G.; Kirchartz, T.; et al. Influence of Crystallinity and Energetics on Charge Separation in Polymer−Inorganic Nanocomposite Films for Solar Cells. Sci. Rep. 2013, 3, 1−8. (21) Zhu, X. Y.; Yang, Q.; Muntwiler, M. Charge-Transfer Excitons at Organic Semiconductor Surfaces and Interfaces. Acc. Chem. Res. 2009, 42, 1779−1787. (22) Jailaubekov, A. E.; Willard, A. P.; Tritsch, J. R.; Chan, W. L.; Sai, N.; Gearba, R.; Kaake, L. G.; Williams, K. J.; Leung, K.; Rossky, P. J.; Zhu, X. Y. Hot Charge-Transfer Excitons Set the Time Limit for Charge Separation at Donor/Acceptor Interfaces in Organic Photovoltaics. Nat. Mater. 2013, 12, 66−73. (23) Tisdale, W. A.; Williams, K. J.; Timp, B. A.; Norris, D. J.; Aydil, E. S.; Zhu, X. Y. Hot-Electron Transfer from Semiconductor Nanocrystals. Science 2010, 328, 1543−1547. (24) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.; Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; et al. Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy. J. Am. Chem. Soc. 2008, 130, 3030−3042. (25) Dimitrov, S. D.; Bakulin, A. A.; Nielsen, C. B.; Schroeder, B. C.; Du, J.; Bronstein, H.; McCulloch, I.; Friend, R. H.; Durrant, J. R. On the Energetic Dependence of Charge Separation in Low-Band-Gap Polymer/Fullerene Blends. J. Am. Chem. Soc. 2012, 134, 18189− 18192. (26) Arkhipov, V. I.; Heremans, P.; Bässler, H. Why is Exciton Dissociation so Efficient at the Interface Between a Conjugated Polymer and an Electron Acceptor? Appl. Phys. Lett. 2003, 82, 4605. (27) Yang, Y.; Rodríguez-Córdoba, W.; Xiang, X.; Lian, T. Strong Electronic Coupling and Ultrafast Electron Transfer between PbS Quantum Dots and TiO2 Nanocrystalline Films. Nano Lett. 2011, 12, 303−309. 211

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