Letter pubs.acs.org/macroletters
Highly Ordered Nanoconfinement Effect from Evaporation-Induced Self-Assembly of Block Copolymers on In Situ Polymerized PEDOT:Tos Yeon Hyeok Lee,†,‡ Jinwoo Oh,† Sang-Soo Lee,†,‡ Heesuk Kim,† and Jeong Gon Son*,† †
Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Organic thermoelectric materials based on conducting polymers have focused on increasing electrical conductivity and optimizing thermoelectric properties via dedoping processes. To control the crystallinity and crystal alignment for enhanced electrical conductivity, a confinement geometry in nanostructures with grapho-epitaxial growth of conducting polymers during in situ polymerization could be a promising approach. We obtained highly ordered lamellar, cylindrical and disordered nanostructures from PEO-b-PPO-bPEO block copolymer (BCP) and iron(III) tosylate (Fe(Tos)3) oxidant blended films and solvent evaporationinduced self-assembly (EISA) processes. Then, in situ vapor phase polymerization of poly(3,4-ethylenedioxythiophene) (PEDOT):Tos on differently ordered oxidant/BCP films was performed. The effect of BCP nanostructures on the crystallinity, crystal orientation and electrical conductivity of the PEDOTs was confirmed by nanostructural and crystallographic analyses using grazing incidence small and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively) experiments before and after polymerization and after a washing process. Different washing solvents also affected the electrical conductance and crystal structure. We achieved thermoelectric thermopowers up to 70 μW·m−1·K−2 by using an immersion dedoping process to reduce the carrier concentration and enhance the Seebeck coefficient, with little change of crystal structure.
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thermoelectric applications because of its superior electrical conductivity, air-stability, and processability compared with other conducting polymers. Using solvent-treated PEDOT:polystyrenesulfonate (PSS)7,8 and vacuum phase in situ polymerization9−13 provided high electrical conductivity, exceeding 1000 S·cm−1. Additionally, by using a dedoping process to control the carrier concentration, higher Seebeck coefficients and optimal thermoelectric properties were achieved, but with a slight reduction in electrical conductivity.14−17 Notably, a structural confinement effect on the crystal orientation has been observed during in situ PEDOT polymerization, for example, for grapho-epitaxial growth from topographical nanopatterns9 and poly(ethylene oxide)-blockpoly(propylene oxide)-block-poly(ethylene oxide) (PEO-bPPO-b-PEO) block copolymer (BCP) self-assembly assisted polymerization.15,18−21 The BCP efficiently blocks the crystallization of the oxidant and facilitates its mobility during the polymerization. Originally, the BCPs provide microphase separation between two blocks and enable the formation of
hermoelectric devices can directly convert a temperature difference into electrical energy via the Seebeck effect, which originates from the difference in the charge carrier densities of the hot and the cold sides.1 To evaluate thermoelectric efficiency, the figure of merit of thermoelectric materials is ZT = S2σT/κ, where S is the Seebeck coefficient, which indicates the voltage arising from the temperature difference, σ is the electrical conductivity, and κ is the thermal conductivity. While inorganic-based thermoelectric materials such as BiTe and PbTe were developed long ago,2 the coming of Internet-of-things (IoT) era and the development of wearable devices has demanded lightweight and flexible organic thermoelectric materials for self-powered system by body heat.3−6 Conducting polymers, graphene, carbon nanotubes, and their composites can be used to develop organic thermoelectric materials. Among them, the conducting polymers that have high electrical conductivities from π-conjugated backbones and low thermal conductivities can be promising candidates for efficient organic thermoelectric materials.4−6 Control of crystallinity, crystal orientation, molecular weight and doping level have been widely studied to enhance the electrical conductivity of conducting polymers. Poly(3,4-ethylenedioxythiophene) (PEDOT) is the representative conducting polymer for © XXXX American Chemical Society
Received: February 21, 2017 Accepted: March 21, 2017
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DOI: 10.1021/acsmacrolett.7b00137 ACS Macro Lett. 2017, 6, 386−392
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Scheme 1. Schematic Images of the In Situ Vapor Phase Polymerization (VPP) of Poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos) with Different Nanostructures of PEO-b-PPO-b-PEO Block Copolymer/Iron(III) Tosylate (Fe(Tos)3) Oxidant Mixture Films from Various Evaporation-Induced Self-Assembly (EISA) Processes
Figure 1. Grazing incidence small-angle X-ray scattering (GISAXS) scattering patterns of Fe(Tos)3 oxidant/P123 BCP mixed films before polymerization with the control for mixing ratios of (a, d) 4:6, (b, e) 5:5, and (c, f) 6:4, and after EISA processing using (a−c) 1-butanol and (d−f) DMF/1-butanol 5:5 mixed solvent.
films. The effect of BCP nanostructures on the crystallinity, crystal orientation and electrical conductivity of these differently synthesized PEDOT:Tos films were analyzed. We mainly used grazing incidence small and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively) to probe the nanostructure and crystallography. These analyses were done before polymerization, after polymerization and after washing with different solvents to remove the oxidant/BCP. The optimized thermoelectric properties were achieved using the synthesized PEDOT:Tos having the highest electrical conductivity; that PEDOT:Tos was subjected to an immersion dedoping process, which enhanced the Seebeck coefficient with minimal change in crystallinity and crystal orientation. Scheme 1 shows the in situ VPP of PEDOT:Tos with P123 PEO-b-PPO-b-PEO BCP/iron tosylate mixed films. Control of
periodic nanostructures such as lamellae, cylinders, gyroids, and spheres via self-assembly.22−27 PEO-b-PPO-b-PEO is a wellknown template material for realizing highly ordered mesoporous carbon or silica nanostructures.28−30 However, the nanostructural effect of BCPs during in situ PEDOT polymerization studies with BCPs has not yet been considered. Only Fabretto et al.19 insisted that lamellar-like PEDOT nanostructures might be from the BCP self-assembly indirectly grounded sheet-like film morphology on AFM. In this study, we obtained highly ordered lamellar, cylindrical and disordered nanostructured films from mixtures of PEO-bPPO-b-PEO BCPs and iron(III) tosylate (Fe(Tos)3) oxidant via a solvent evaporation-induced self-assembly (EISA) process29−32 and performed in situ vapor phase polymerization (VPP) of PEDOT:Tos on differently ordered oxidant/BCP 387
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oxidant/BCP blended film had enlarged volumes of PEO domains, and a lamellar morphology for the 4:6 samples and a cylindrical morphology with a PEO majority block for the 5:5 and 6:4 films under the DMF/1-butanol EISA condition. Noting the diverse nanostructures observed for the blend films, PEDOT:Tos films were then synthesized via VPP. The nanostructures of the blend films were essentially maintained after the polymerization process, as confirmed by GISAXS experiments (Figure S2). The electrical conductivities of the PEDOT:Tos films were measured following washing of the films with methanol. Figure 2 shows the results as a function of
the mixing ratio and EISA condition provided highly ordered lamellae, cylinders and disordered nanostructures. Different oxidant/BCP mixture ratios, ranging from 4:6 to 6:4 with small amounts of added pyridine (to reduce the reactivity of the oxidant), were dissolved in 1-butanol or DMF/1-butanol (5:5) mixed solvent for the different EISA conditions. The solutions were spin-coated with a considerable excess of solvent for 5 s just to provide uniform dispersions of the coating solutions on the substrates. The solvent-containing oxidant/BCP films were placed on a glass Petri dish lid and dried on a hot plate at 60 °C to retard solvent evaporation and induce self-assembly of the BCPs with the oxidant. Highly ordered lamellar or cylindrical structures were obtained at different oxidant/BCP ratios after the EISA processing with the mixed solvent, while 1-butanoltreated films were mainly phase-disordered. The films were stored at 60 °C to avoid crystallization of the Fe(Tos)3 oxidant. Figure S1 in the Supporting Information shows images of Fe(Tos)3 in an oxidant/BCP film that crystallized below 30 °C and indications of crystallization after the polymerization. The EISA-treated films were polymerized with EDOT vapor in a small vacuum chamber at 60 °C. The polymerized films were washed with several different solvents (immersing for 5 min and rinsing for 30 s) to remove the BCP and oxidant from the PEDOT:Tos films. During the washing process, the thickness of the films significantly decreased from about 8 μm to 120 nm for 6:4 blended films and 80 nm for 4:6 blended films on both EISA conditions, which was less than 1.5% of the original oxidant/BCP film thickness. GISAXS was used for nanostructural analysis of the oxidant/ BCP films after EISA processing (Figure 1). To avoid crystallization of the Fe(Tos)3 during the scattering experiments, the sample temperature was maintained at 60 °C from the EISA process to the GISAXS sample stage. Because of the grazing incidence angle of about 0.08° and relatively thick films of about 8 μm, transmitted diffraction peaks were predominantly observed by the two-dimensional detector. Figure 1a−c shows the scattering patterns of the 4:6, 5:5, and 6:4 oxidant/ BCP mixed films prepared under the 1-butanol EISA condition. Isotropic ring patterns at about 0.058 Å−1 were observed for all oxidant/BCP blending ratios, which indicated that the oxidant/ BCP blends microphase separated with a period of about 10.8 nm, but did not form any ordered structures. Increasing the Fe(Tos)3 content in the blend films caused the isotropic rings to gradually smear, which indicated diminishing microphase separation. However, with the DMF/1-butanol EISA processing, the oxidant/BCP films showed highly ordered scattering. In the case of the 4:6 blend ratio (Figure 1d), the scattering patterns were only arrayed on the qz axis at qz = ∼0.06, 0.12, and 0.18 Å−1. This indicated parallel-oriented lamellar structures with a period of about 10.5 nm. The 5:5 and 6:4 blended samples (Figure 1e and f, respectively) show hexagonal (10) peaks around the transmitted beam and higher order (11), (21), and (30) peaks. This indicated highly ordered and parallel-oriented cylindrical structures with domain spacings of about 11.9 and 10.8 nm. The volume fraction of PEO in the P123 PEO-b-PPO-b-PEO BCPs was about 30%, which can form a cylindrical morphology with the PEO minority block in the molten state. The Fe(Tos)3 has hydrophilic properties derived from the polar Fe3+ and three tosylate (Tos−) ions and PEO is more hydrophilic than PPO. Therefore, small polar molecules of Fe(Tos)3 can mainly segregate into the hydrophilic PEO domains of the BCP, and thereby change the volume ratio between the PEO and PPO domains. The
Figure 2. Electrical conductivities of PEDOT:Tos films made using different weight ratios of Fe(Tos)3 oxidant:P123 BCP and processed under different EISA conditions with 1-butanol (red dots) or DMF/1butanol 5:5 mixed solvent (blue dots).
a Fe(Tos)3:P123 BCP blending ratio ranging from 3:7 to 7:3, and the effect of using 1-butanol only or the DMF/1-butanol mixed solvent in the EISA process. For all blend ratios, the electrical conductivity of the PEDOT:Tos films having disordered nanostructures using 1-butanol EISA condition were much higher than those from the highly ordered nanostructures obtained using DMF/1-butanol. Increasing the Fe(Tos)3 oxidant fraction in the blend films improved the electrical conductivity until the 6:4 ratio, and the highest electrical conductivity of over 2200 S cm−1 was measured for the 6:4 blending ratio of PEDOT:Tos film that used the 1butanol EISA process. Reduction in conductivity of high loading BCP films may be originated from residual BCPs after the washing process which possess the space in the film without contribution of electrical conductivity,33 but the Fe(2p) and O(1s) XPS spectra in Figure S3 did not show any significant amount of residuals in the washed films. The residual DMF in oxidant/BCP films also can affect to the in situ synthesis of PEDOT:Tos films,34 but after the evaporation-induced EISA process, the thickness of the synthesized PEDOT:Tos films was almost the same. GIWAXS experiments were conducted to understand, from the perspective of crystal growth and orientation, why the disordered oxidant/BCP films exhibited superior electrical conductivities. The polymerized samples prepared under the two EISA conditions were examined before and after the washing process (Figure 3). Before washing, all GIWAXS patterns displayed a sharp isotropic ring at about 0.5 Å−1 and broad rings like rings-of-Saturn at 1.2, 1.4, 1.6, and 1.9 Å−1. Only 6:4 blended films with both EISA conditions showed clear scattering spot at 0.44 Å−1 along the out-of-plane direction while 4:6 blended films did not show such a peak, as can be 388
DOI: 10.1021/acsmacrolett.7b00137 ACS Macro Lett. 2017, 6, 386−392
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Figure 3. (a−d) Grazing incidence wide-angle X-ray scattering (GIWAXS) scattering patterns of polymerized PEDOT:Tos films before washing for (a, c) 4:6 and (b, d) 6:4 Fe(Tos)3:P123 films after (a, b) 1-butanol and (c, d) DMF/1-butanol EISA processing. (e−h) GIWAXS patterns of polymerized PEDOT:Tos films after washing of (e, g) 4:6 and (f, h) 6:4 Fe(Tos)3:P123 films under (e, f) 1-butanol and (g, h) DMF/1-butanol EISA processing.
crystals. The broad out-of-plane peak near 1.87 Å−1 is originated from less ordered amorphous matrix. In the case of the 4:6 blend films (Figure 3e), similar out-of-plane (100) and (200) peaks of relatively low intensities were evident. However, the pattern did not show any in-plane peak at 1.87 Å−1 and only showed very broad π−π spacing near out-of-plane direction of 1.87 Å−1. This means the π−π stacking is slightly oriented parallel to the substrate (face-on packing) or less ordered in amorphous matrix. We believe the lower concentration of oxidant in the 4:6 blend films led to insufficiently supplied oxidant and sparsely synthesized PEDOT chains in the matrix, which resulted in less packed PEDOT:Tos chains or less portion of PEDOT crystallite domains. The crystal structures of the DMF/1-butanol-treated PEDOT:Tos films were different from those of the 1butanol-treated samples. The polymerized PEDOT:Tos in the lamellar 4:6 blend film nanostructures (Figure 3g) gave the strongest (020) peak and relatively weak (100) and (200) peaks in the out-of-plane direction which means formation of more face-on packing of PEDOT:Tos crystals. The scattering patterns from the cylindrical nanostructures of the 6:4 blend (Figure 3h) indicate less face-on packing from the (020) peaks and enhanced edge-on packing from the (100) and (200) peaks that originated from the higher concentration of oxidant and less confined cylindrical structures. Based on the GIWAXS and electrical conductivity results, the PEDOT:Tos crystal packing direction is closely related with the electrical conductivity. According to G. Hadziioannou et al.,12 charge transport in conducting polymers is determined by the presence of a highly conductive crystal domains embedded in a low-conductivity amorphous polymer matrix and affected by orientation of the crystalline layer with respect to the plane of measurement. The charge mobility along the polymer back-
seen in inset images of Figure 3a−d. The ring patterns were attributed to residual iron tosylate and PEO crystals,13 but the out-of-plane spot of the 6:4 blended films are oriented from the highly aligned PEDOT:Tos crystallite.12 The spot is 14.3 Å of periodicity and correspondence with (100) peak from orthorhombic crystal structures of PEDOT:Tos which direction in a unit cell is coincident with elongated EDOT monomer direction and perpendicular against polymer backbone.12 Because this intense spot was in the out-of-plane direction, alternating layered structures between standing-up EDOT units; this is referred to as edge-on packing in Scheme 1.10 Considering that strong out-of-plane scattering spots only appear in 6:4 films regardless of the nanostructures of the BCPs, it appears that the alignment of PEDOT crystal structure is already formed before the washing process and mainly affected by the surface of the film. Figure S4 shows the GIWAXS peaks of P123 BCP and Fe(Tos)3. After washing, while the GISAXS patterns are all similar in Figure S5, GIWAXS of the four samples in Figure 3e−h were quite different. The disordered nanostructures of the 1-butanoltreated 6:4 blended films (Figure 3f) showed already observed intense scattering peak at 0.44 Å−1 and its high order peaks at 0.89, 1.37, and 1.75 Å−1 and a broad, weak peak at 1.87 Å−1 along the out-of-plane direction. The strong spots are correspondence with (100) and high order (200), (300), and (400) peaks from edge-on directionally and highly aligned orthorhombic crystal structures of PEDOT:Tos. These high order peaks were obscured by dominant Fe(Tos)3 and BCPs before the washing process, but appeared after the washing. The peaks located at 1.87 Å−1 along the in-plane direction corresponded to the (020) peak from π−π stacking of delocalized rings in the EDOT monomeric unit in which the rings are perpendicularly aligned to the substrate. This result is also strong evidence of edge-on packing of the PEDOT:Tos 389
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Figure 4. (a) Conductance changes (sheet resistance ratio) for 6:4 oxidant/BCP blend and 1-butanol EISA-treated PEDOT:Tos films before and after washing with various solvents. (b) GIWAXS scattering patterns of PEDOT:Tos films washed with various solvents.
bone and along the π−π stacking direction is much higher than the mobility measured across the lamellae of crystal plane. Therefore, for the higher in-plane electrical conductivity in thin films, the edge-on orientation is essential that the π−π stacking and chain propagation exist horizontally in the films to facilitate interchain and intrachain hopping transport. Collectively, higher electrical conductivity and more highly aligned crystal structures of PEDOT:Tos films could be obtained from the disordered, rather than the ordered, nanostructures. We initially intended to form highly ordered nanostructures of oxidant/BCP films and provide a highly ordered confined geometry for better alignment of crystal structures. However, unlike a hard template such as topographical nanopatterns,9 the PEO-b-PPO-b-PEO BCP nanostructures, which has a glass transition temperature lower than room temperature, is a movable and soft template and has a interchangeable boundary, which made it difficult to induce epitaxial growth of PEDOT:Tos from the BCP nanostructures. Furthermore, sharp phase separation in the highly ordered nanostructures efficiently disturbed the penetration of Fe(Tos)3 oxidant or EDOT monomers into the blend films during the VPP, which made it difficult to form sufficiently large and highly aligned PEDOT crystal domains. Based on these results, we concluded that the disordered states of oxidant/BCP blends were better suited to achieve PEDOT:Tos films having high electrical conductivities and highly aligned crystal structures. Changes in the crystal structure occurred during the washing process. Thus, we also examined the effect of the washing solvent on the electrical conductivities and crystal structures of the PEDOT:Tos films. Figure 4a compares the rate of change of conductance (sheet resistance ratio before and after the washing) observed for the various washing solvents for the 6:4 oxidant/BCP blend subjected to the 1-butanol EISA treatment. Relatively hydrophilic solvents, such as alcohols, caused less conductance change while relatively hydrophobic solvents, such as chloroform and tetrahydrofuran (THF), considerably lowered the conductance, by up to 80%. The GIWAXS scattering patterns (Figure 4b) for methanol-, ethanol-, and acetone-washed films show strong out-of-plane (100), (200), and (300) and in-plane (020) peaks that correspond to highly ordered edge-on packing crystal structures, while the relatively hydrophobic DMF or NMP solvents display weaker (100) and (200) peaks and also a blurred (020) peak along the out-of-
plane direction that indicates the coexistence of edge-on and face-on packing of PEDOT:Tos crystals. These results can be explained as follows: hydrophilic solvents better maintain the crystal structures and coordinated tosylate ions during the vertical shrinkage, and hydrophobic solvents can more easily change the conformation of the PEDOT chains. Using these conductivity-maximized PEDOT:Tos films, we carried out a dedoping process to control the oxidation level and optimize the thermoelectric properties. Several dedoping processes have been reported, including electrochemical methods,15 chemical reduction using TDAE,14 and pH control;16 we chose a solvent immersion method for a long time using slow and spontaneous diffusion of doped tosylate ions. Figure 5a shows the S(2p) X-ray photoelectron spectroscopy (XPS) spectra of PEDOT:Tos films for different immersion times in methanol and in DMF for 1 h. The sulfur atom in the thiophene part of the polymer chain had a relatively
Figure 5. (a) S(2p) XPS spectra of PEDOT:Tos films after the immersion dedoping process with methanol from initial washing to 5 h of immersion and with DMF for 1 h. (b) Calculated oxidation levels of PEDOT:Tos films under different immersion times in methanol. (c, d) GIWAXS scattering patterns of PEDOT:Tos films after 5 h of immersion in (c) methanol and (d) DMF. 390
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Figure 6. (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor of polymerized PEDOT:Tos films as a function of immersion time from initial washing to 5 h of immersion in methanol and DMF of 1-butanol EISA-treated 6:4 blended films.
determined by GISAXS/GIWAXS and four-point probe experiments. Higher electrical conductivities and more highly aligned crystal structures could be achieved for the PEDOT:Tos films from disordered nanostructures rather than ordered nanostructures because of easier mass transport of oxidant and monomers in the disordered nanostructures. Washing with different solvents strongly affected the electrical conductance and crystal structure. With the conductivitymaximized PEDOT:Tos films, we optimized the Seebeck coefficient and electrical conductivity using an immersion dedoping process to reduce carrier concentration, and maximized the thermoelectric thermopower to 70 μW·m−1·K−2.
low binding energy (161−163 eV), while the sulfur atom in the tosylate ion had a higher binding energy (164−167 eV) because of the presence of three oxygen atoms. The oxidation level can be obtained by comparing the areal ratio of S(2p)tosylate/ S(2p)thiophene; the change of oxidation level as a function of immersion time is presented in Figure 5b.14 The oxidation level of films immediately after washing was about 34% but the oxidation level decreased to 16% after 3 h of immersion, whereupon the value remained constant. Crystal structural changes were observed by GIWAXS for PEDOT:Tos films immersed for 5 h in methanol and DMF. The scattering patterns indicated that the out-of-plane (100) and (200) peaks remained but were slightly blurred, and the in-plane (020) peak had changed to an approximately isotropic ring, which indicated that the crystal structure of PEDOT:Tos was essentially maintained even after the dedoping of tosylate ions via the immersion dedoping process. The electrical conductivity and thermoelectric properties of the PEDOT:Tos films, after the dedoping process with various solvents, and immersion time are shown in Figure 6. With increasing immersion time, the electrical conductivity decreased with two solvents: from over 2200 S·cm−1 to about 850 and 500 S·cm−1 in methanol and DMF, respectively. The thermoelectric Seebeck coefficient tended to increase with time in the same order and eventually saturated within 1 h for DMF and about 3 h for methanol. The saturated Seebeck coefficients were 28 and 24 μV·K−1, respectively. The power factors σS2, which is a measure of the thermoelectric efficiency calculated from the electrical conductivity and Seebeck coefficient, are given in Figure 6c. In the case of methanol, the power factor increased until 2 h to a maximum value of about 70 μW·m−1·K−2 and then decreased. However, the DMF showed peak at 1 h but less value of the power factor because of rapid dedoping and less ordered crystal structures. Using the immersion dedoping process, we were able to obtain the optimized thermoelectric properties of PEDOT:Tos films from the structurally optimized synthetic condition. Summarizing, we obtained highly ordered lamellar, cylindrical and disordered nanostructures from PEO-b-PPO-b-PEO BCP and Fe(Tos)3 oxidant blended films and a solvent EISA process. This provided a confined geometry effect in the nanostructures during in situ PEDOT polymerization. After VPP of PEDOT:Tos on differently ordered oxidant/BCP films, the effect of the BCP nanostructure on the crystallinity, crystal orientation, and electrical conductivity of PEDOTs was
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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/acsmacrolett.7b00137. Experimental method and optical images of PEDOT:Tos films, GISAXS scattering patterns of PEDOT:Tos films before washing and GIWAXS patterns of P123 block copolymer and Fe(Tos)3 (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jeong Gon Son: 0000-0003-3473-446X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Global Frontier Research Program (2011-0032156) funded by the Korean Government (MEST), the R&D Convergence Program of NST (National Research Council of Science and Technology) of the Republic of Korea, and the Korea Institute of Science and Technology (KIST) Internal Project. The experimental support by the staffs at the 3C beamline of the Pohang Light Source is also gratefully acknowledged.
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REFERENCES
(1) Bell, L. E. Science 2008, 321 (5895), 1457−1461. (2) Heremans, J. P.; Dresselhaus, M. S.; Bell, L. E.; Morelli, D. T. Nat. Nanotechnol. 2013, 8 (July), 471−473.
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DOI: 10.1021/acsmacrolett.7b00137 ACS Macro Lett. 2017, 6, 386−392
Letter
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DOI: 10.1021/acsmacrolett.7b00137 ACS Macro Lett. 2017, 6, 386−392
