Aerosol-Assisted Fine-Tuning of Optoelectrical Properties of SWCNT

Jun 24, 2019 - Chegel, R. Third-Nearest-Neighbors Tight-Binding Description of Optical Response of Carbon Nanotubes: Effects of Chirality and Diameter...
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Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3961−3965

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Aerosol-Assisted Fine-Tuning of Optoelectrical Properties of SWCNT Films Alexey P. Tsapenko,*,†,‡ Stepan A. Romanov,† Daria A. Satco,† Dmitry V. Krasnikov,† Pramod M. Rajanna,†,‡ Mati Danilson,§ Olga Volobujeva,§ Anton S. Anisimov,∥ Anastasia E. Goldt,† and Albert G. Nasibulin*,†,‡ †

Skolkovo Institute of Science and Technology, Nobel Str. 3, 121205 Moscow, Russian Federation Aalto University, 00076 Espoo, Finland § Tallinn University of Technology, Ehitajate tee 5, 12616 Tallinn, Estonia ∥ Canatu Ltd., Konalankuja 5, 00390 Helsinki, Finland

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‡

S Supporting Information *

ABSTRACT: We propose a novel, scalable, and simple method for aerosol doping of single-walled carbon nanotube (SWCNT) films. This method is based on aerosolization of a dopant solution (HAuCl4 in ethanol) and time-controlled deposition of uniform aerosol particles on the nanotube film surface. The approach developed allows fine-tuning of the SWCNT work function in the range of 4.45 (for pristine nanotubes) to 5.46 eV, controllably varying the sheet resistance of the films from 79 to 3.2 Ω/□ for the SWCNT films with 50% transmittance (at 550 nm). This opens a new avenue for traditional and flexible optoelectronics, both to replace existing indium−tin oxide electrodes and to develop novel applications of the highly conductive transparent films.

ransparent and conductive films (TCFs) based on singlewalled carbon nanotubes (SWCNTs) are considered to be a promising material for next-generation optoelectronic devices.1−6 In contrast to the traditional rigid n-type transparent conductive metal oxides (e.g., indium−tin oxide), SWCNT films are flexible and stretchable and usually possess hole charge carriers.7−10 The main barrier hindering a wide range of SWCNT applications in optoelectronics is poor control of SWCNT electronic properties (e.g., conductivity and Fermi level).11−14 Some optoelectronic applications, like multijunction solar cells or water-splitting devices, require both p- and n-type transparent electrodes with controlled properties, namely, a precisely aligned band structure, to achieve the best performance.15−17, To increase the conductivity of the SWCNTs, usually one of three doping methods is utilized: drop-casting, spin-coating, or dip-coating.4,18 Although these techniques allow one to significantly decrease sheet resistance of pristine SWCNT films (up to 15 times),19 they lack spatial uniformity and scalability due to nonuniform evaporation of liquid solvent, leading to a coffee-ring effect.4 Moreover, none of the methods allow one to precisely control the Fermi level in the SWCNTs. Here, we propose a novel technique to dope SWCNT films in a controlled manner by employing liquid aerosol particles (droplets) containing a dopant dissolved in a volatile solvent. By varying the aerosol deposition time, we carefully control the work function and sheet resistance of SWCNT films. The main

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advantages of the technique proposed over the widely used spin-coating20 deposition method are a continuous regime that can be easily scaled up for roll-to-roll manufacturing lines and economical consumption of the dopant solution. Furthermore, it allows one to work with both freestanding SWCNT films and films on a substrate.12 Also, the proposed method utilizes a doping solution with a single concentration by controlling only one parameter, the deposition time (opposite of spin-coating and drop-casting counterparts), thereby saving resources during doping of the SWCNT films. We believe that the method proposed allows one to open a new avenue for finetuning of the electronic structure of low-dimensional materials for optoelectronics not only to replace the existing indium−tin oxide electrodes but also to develop novel applications of highly conductive, flexible, and transparent films. For our experiments, we utilized high-quality films of randomly oriented SWCNTs produced by the aerosol (floating catalyst) chemical vapor deposition method.21 We employed thin SWCNT films with 50 and 85% transmittance (at a wavelength of 550 nm) as the model samples. The assynthesized SWCNT films were dry transferred onto a required substrate (usually quartz or PET; a free-standing Received: May 24, 2019 Accepted: June 24, 2019 Published: June 24, 2019 3961

DOI: 10.1021/acs.jpclett.9b01498 J. Phys. Chem. Lett. 2019, 10, 3961−3965

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The Journal of Physical Chemistry Letters configuration was obtained by transferring the SWCNT films over an opening in the substrate)22 for dopant deposition and further characterization. The detailed description of the experimental setup is presented in the Supporting Information. Briefly, one of the most effective p-type dopants, namely, tetrachloroauric(III) acid trihydrate (HAuCl4·3H2O; ACROS Organics), was dissolved in ethanol (C2H5OH; 99.5%, ETAX) at a concentration of 3.75 or 7.50 mM and aerosolized by an OMRON compressor nebulizer NE-C801 (Figures 1 and S1).

Figure 1. (a) Experimental setup used for aerosol doping of SWCNT films by 11 μm sized droplets. (b) Number size distribution of residue aerosol particles left after evaporation of pure ethanol (red) and a solution of HAuCl4 in ethanol (black). The total concentration (Ctot), geometric mean diameter (GMD), and geometric standard deviation (GSD) are presented for each of the distributions.

The aerosol was transferred with an air flow of 6 L/min through a tube with a diameter of 12 mm and deposited on top of SWCNT films. Ethanol droplets (∼11 μm in size, as estimated from the number size distribution measurements and droplet traces in Figures S2 and S3) were deposited on the surface of SWCNTs. The droplets impregnated the SWCNT films with subsequent ethanol evaporation, leaving a HAuCl4 coating on the surface (Figure 2). It is worth mentioning that the droplets of the dopant solution after complete solvent evaporation formed aerosol residue particles23,24 with a diameter of 70 nm as measured by Scanning Mobility Particle Sizer Spectrometer 3938 (TSI) and described in the Supporting Information (Aerosol doping procedure). The quantity of the dopant was estimated to be in good agreement with the HAuCl4 amount in one droplet deposited on the film (Figures S2 and S3). Figure 2 shows scanning (SEM, HR-SEM Merlin, Zeiss) and transmission (TEM, Tecnai G, FEI) electron microscopy images of the samples of pristine and doped SWCNTs. Considering the morphology evolution with the doping time, we achieved a more densified film for the highly doped sample (Figure 2a) as well as larger coating and filling with Au nanoparticles (NPs) across the film (Figure 2b,c). The origin of gold NPs can be accounted for the spontaneous reduction of Au3+ species to Au0, leading to charge transfer to SWCNTs.20 As previously shown, HAuCl4 is one of the most efficient dopants for SWCNTs, which shifts the Fermi level deeper in the valence band.19 The shift mainly depends on the concentration of the dopant that appeared on the surface of SWCNTs. Therefore, increasing the deposition time (1.5, 9.0, and 1050 s) results in a higher doping level in the SWCNTs, which can be observed by a smooth spectral change with the doping time in the UV−vis−NIR (Figure 3a−e) and Raman (Figure S8a−d) spectra and measurements of the sheet

Figure 2. Typical (a) SEM and (b,c) TEM images of pristine (0.0 s) and doped SWCNTs at doping times of 1.5, 9.0, and 1050 s. Circles indicate large and small black spots corresponding to the catalyst and Au NPs.

resistance and work function of the films (Figure 3g,h). Increasing the doping time results in a decrease of the sheet resistance from 79 to 3.2 Ω/□ for the SWCNT films with 50% transmittance (at 550 nm). The mentioned increase in the conductivity is consistent with the work function rise from 4.45 for the pristine nanotubes to 5.46 eV for the doping time of 1050 s (Figure 3f). The dramatic changes in the sheet resistance (24.5 times higher compared to the pristine values) can be explained by the efficient doping method and are attributed to both reduction of the Schottky barrier between the metallic and semiconducting nanotubes25 and the increase in the concentration of charge carriers in the SWCNTs.26 To gain a detailed insight into the transformation of optical absorbance spectra under doping and to correlate it, thereby, with the Fermi level position, we performed peak fitting for the spectra of pristine and doped SWCNT samples (Figure 3b−e). The experimental data were fitted with a model based on a linear combination of Lorentzian contours and exponential decay, discussed in detail in the Supporting Information (Optical spectra calculation methods). As a result, in the case of pristine nanotubes, we clearly observed all van Hove singularity transitions in semiconducting nanotubes from ES11 M to ES55 and in metallic SWCNTs from EM 11 to E44 (Figure 3a). For the highly doped sample (1050 s), the low-energy transitions disappeared, but three new peaks can be clearly distinguished at 230, 315, and 1318 nm (Figure 3b). The origin of the newborn peak at the higher wavelength is attributed to the intersubband plasmon.27 The peaks at 230 3962

DOI: 10.1021/acs.jpclett.9b01498 J. Phys. Chem. Lett. 2019, 10, 3961−3965

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the SWCNTs on the diameter, the values of the Fermi level position can be estimated in a certain range from the disappearance of van Hove singularity transitions. The work function of the pristine SWCNTs, as previously measured by the ultraviolet photoemission spectroscopy technique, was found to be Φ 0.0 = 4.45 eV.19 In principle, the Fermi level position in SWCNTs in air laboratory conditions can be shifted up to 0.7 eV25,28 due to the ambient doping by oxygen. However, annealing of the SWCNT film allowed us to recover the Fermi level to its intrinsic position (Methods and Results section in the Supporting Information). For the sample treated for 1.5 s, the work function shifted to Φ 1.5 = 4.82 ± 0.15 eV. Indeed, we observe the semiconducting ES11 transition (Φ SV1 = 4.71 eV) to disappear, while the ES22 transition (Φ SV2 = 4.92 eV) is still present. At the doping time of 9.0 s, the work function can be estimated to be in line with the disappearance of ES22 M and the still observable metallic EM 11 transition (Φ V1 = 5.15 eV): S S Φ 9.0 = 5.03 ± 0.15 eV. At 1050 s, we have no EM 11 and E33 (ΦV3 S S = 5.35 eV) transitions but still observe the E44 transition (Φ V4 = 5.57 eV), leading to Φ 1050 = 5.46 ± 0.15 eV. For clarity of the picture, we plot the van Hove singularity transitions with corresponding Fermi level shift in the density of states (DOS) of typical 2 nm in diameter semiconducting and metallic nanotubes (Figure 3f) calculated within the thirdnearest-neighbor tight-binding mode.29 While the doping time and corresponding amount of dopant (Figure S9) increase, the van Hove singularity transitions disappear one by one (Figure 3a,f). All of this combined allowed us to estimate the work function of the doped SWCNTs. Additionally, the aerosol doping technique can be applied to SWCNT films with different thicknesses to demonstrate the applicability of the method for various needs: while the thin films are of huge importance for transparent and conductive electrodes,30 the thick ones can be used for other applications that do not require high transmittance (e.g., back contacts for solar cells,4,22 electrochemical electrodes,4,22 gas sensors,4,22 thermoacoustic generators,31 etc.). However, the deposition parameters must be subsequently adjusted. In the case of thinner films with a transmittance of 85%, to improve control of the doping level, we diluted the dopant concentration by a factor of 2. This resulted in a 13.5-fold decrease in the sheet resistance from 485 to 36 Ω/□ (see Figure S7). For thinner SWCNT films, we observe a similar evolution of optical spectra with the doping level. It is characterized by the gradual disappearance of low-energy peaks and excitation of the intersubband plasmon27 as well as the appearance of higherenergy plasmon peaks likely associated with gold NPs.32 Apparently, the method that we introduced can be easily applied to bendable and elastic substrate broadening, thereby, the range of applications of the developed TCFs to the field of flexible and stretchable devices.33 The demonstrated method may also be applied to other low-dimensional materials as well as their hybrid structures. These can yield numerous applications such as direct film micropatterning, including drawing conductive paths for wiring, and stack assembly for creation of multilayered functional materials.34−36 In summary, we developed a novel aerosol approach for uniform, controllable, and reproducible doping of SWCNT films. This method allowed us to tune the work function and conductivity of the nanotubes by varying the deposition time of liquid aerosol particles containing HAuCl4 dissolved in ethanol. As a result, we modulated the work function of pristine films with 50% transmittance (at 550 nm) in the range

Figure 3. (a) UV−vis−NIR absorbance color map and corresponding absorbance spectra of the 50% transmittance samples (with fitted peaks) at different doping times: (b) 1050; (c) 9.0; (d) 1.5; and (e) 0.0 s. (f) DOS of (20,9) semiconducting (left) and (25,1) metallic (right) carbon nanotubes corresponding to the different doping times for the Fermi level position: 1050 (green); 9.0 (blue); 1.5 (red); 0.0 s (black). Time dependence of the optoelectronic characteristics during the doping: (g) sheet resistance, Rs; (h) work function, Φ .

and 315 nm are most likely plasmons of gold NPs (Figures S5 and S6).19 The work function (Φ ) of the doped SWCNTs can be calculated from peak fitting of the absorbance spectra. As we deal with not a single-chirality sample but with a distribution of 3963

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Nanotube-Based Flexible Electrothermal Film Heaters with a High Heating Rate. R. Soc. Open Sci. 2018, 5 (6), 172072. (7) Yu, L.; Shearer, C.; Shapter, J. Recent Development of Carbon Nanotube Transparent Conductive Films. Chem. Rev. 2016, 116 (22), 13413−13453. (8) Murat, A.; Rungger, I.; Sanvito, S.; Schwingenschlögl, U. Mechanism of H 2 O-Induced Conductance Changes in AuCl 4 -Functionalized CNTs. J. Phys. Chem. C 2015, 119 (17), 9568−9573. (9) Shi, Y.; Kim, K. K.; Reina, A.; Hofmann, M.; Li, L.-J.; Kong, J. Work Function Engineering of Graphene Electrode via Chemical Doping. ACS Nano 2010, 4 (5), 2689−2694. (10) Mustonen, K.; Laiho, P.; Kaskela, A.; Susi, T.; Nasibulin, A. G.; Kauppinen, E. I. Uncovering the Ultimate Performance of SingleWalled Carbon Nanotube Films as Transparent Conductors. Appl. Phys. Lett. 2015, 107 (14), 143113. (11) Kim, K. K.; Kim, S. M.; Lee, Y. H. Chemically Conjugated Carbon Nanotubes and Graphene for Carrier Modulation. Acc. Chem. Res. 2016, 49, 390−399. (12) Li, X.; Guard, L. M.; Jiang, J.; Sakimoto, K.; Huang, J. S.; Wu, J.; Li, J.; Yu, L.; Pokhrel, R.; Brudvig, G. W.; et al. Controlled Doping of Carbon Nanotubes with Metallocenes for Application in Hybrid Carbon Nanotube/Si Solar Cells. Nano Lett. 2014, 14 (6), 3388− 3394. (13) Biswas, C.; Candan, I.; Alaskar, Y.; Qasem, H.; Zhang, W.; Stieg, A. Z.; Xie, Y. H.; Wang, K. L. Layer-by-Layer Hybrid Chemical Doping for High Transmittance Uniformity in Graphene-Polymer Flexible Transparent Conductive Nanocomposite. Sci. Rep. 2018, DOI: 10.1038/s41598-018-28658-6. (14) Do, J.-A.; Estrada, D.; Xie, X.; et al. Nanosoldering Carbon Nanotube Junctions by Local Chemical Vapor Deposition for Improved Device Performance. Nano Lett. 2013, 13, 5844−5850. (15) Luo, M.; Liu, Y.; Huang, W.; Qiao, W.; Zhou, Y.; Ye, Y.; Chen, L.-S. Towards Flexible Transparent Electrodes Based on Carbon and Metallic Materials. Micromachines 2017, 8 (1), 12. (16) He, L.; Tjong, S. C. Nanostructured Transparent Conductive Films: Fabrication, Characterization and Applications. Mater. Sci. Eng., R 2016, 109, 1−101. (17) Liu, C.-H.; Zheng, J.; Colburn, S.; Fryett, T. K.; Chen, Y.; Xu, X.; Majumdar, A. Ultrathin van Der Waals Metalenses. Nano Lett. 2018, 18 (11), 6961−6966. (18) Shim, D.; Jung, S. H.; Han, S. Y.; Shin, K.; Lee, K. H.; Han, J. H. Improvement of SWCNT Transparent Conductive Films via Transition Metal Doping. Chem. Commun. 2011, 47, 5202. (19) Tsapenko, A. P.; Goldt, A. E.; Shulga, E.; Popov, Z. I.; Maslakov, K. I.; Anisimov, A. S.; Sorokin, P. B.; Nasibulin, A. G. Highly Conductive and Transparent Films of HAuCl4-Doped SingleWalled Carbon Nanotubes for Flexible Applications. Carbon 2018, 130, 448−457. (20) Kim, S. M.; Kim, K. K.; Jo, Y. W.; Park, M. H.; Chae, S. J.; Duong, D. L.; et al. Role of Anions in the AuCl3-Doping of Carbon Nanotubes. ACS Nano 2011, 5 (2), 1236−1242. (21) Moisala, A.; Nasibulin, A. G.; Brown, D. P.; Jiang, H.; Khriachtchev, L.; Kauppinen, E. I. Single-Walled Carbon Nanotube Synthesis Using Ferrocene and Iron Pentacarbonyl in a Laminar Flow Reactor. Chem. Eng. Sci. 2006, 61 (13), 4393−4402. (22) Nasibulin, A. G.; Kaskela, A.; Mustonen, K.; Anisimov, A. S.; Ruiz, V.; Rackauskas, S.; Timmermans, M. Y.; Pudas, M.; Aitchison, B.; et al. Multifunctional Free-Standing Single-Walled Carbon Nanotube Films. ACS Nano 2011, 5, 3214−3221. (23) Kulkarni, P.; Baron, P. A.; Willeke, K. Aerosol Measurement: Principles, Techniques, and Applications, 3rd ed.; 2011. (24) Agranovski, I. Aerosols - Science and Technology; 2010. (25) Kim, K. K.; Bae, J. J.; Park, H. K.; Kim, S. M.; Geng, H.-Z.; Park, K. A.; Shin, H.-J.; Yoon, S.-M.; Benayad, A.; Choi, J.-Y.; et al. Fermi Level Engineering of Single-Walled Carbon Nanotubes by AuCl3 Doping. J. Am. Chem. Soc. 2008, 130 (38), 12757−12761. (26) Kaskela, A.; Nasibulin, A. G.; Timmermans, M. Y.; Aitchison, B.; Papadimitratos, A.; Tian, Y.; Zhu, Z.; Jiang, H.; Brown, D. P.; Zakhidov, A.; et al. Aerosol-Synthesized SWCNT Networks with

of 4.45−5.46 eV. The corresponding sheet resistance of the highly doped SWCNT film was found to be as low as 3.2 Ω/□. For the 85% films, the sheet resistance dropped from 485 to 36 Ω/□.

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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/acs.jpclett.9b01498.

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Materials; aerosol doping procedure; methods and results; and optical spectra calculation methods (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alexey P. Tsapenko: 0000-0001-5772-7874 Stepan A. Romanov: 0000-0002-5672-0694 Olga Volobujeva: 0000-0002-3844-2555 Albert G. Nasibulin: 0000-0002-1684-3948 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Russian Science Foundation No. 17-19-01787 (synthesis, measurements and characterization of the thin films). A.P.T. acknowledges Russian Foundation for Basic Research No. 18-32-00246 (optical measurements and analysis of the samples) for partial financial support. M.D. acknowledges the Estonian Ministry of Education and Research (IUT19-28) and the European Union through the European Regional Development Fund Project TK141 (photoelectron spectroscopy measurements). P.M.R. is grateful for partial financial support from the Doctoral Studies Internationalization Program Dora Plus activity 2.2 financed by the European Regional Development Fund (photoelectron spectroscopy analysis). This work was partially supported by Skoltech NGP Program.

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REFERENCES

(1) Zhang, L.; Ji, Y.; Qiu, Y.; Xu, C.; Liu, Z.; Guo, Q. Highly Thermal-Stable and Transparent Silver Nanowire Conductive Films via Magnetic Assisted Electrodeposition of Ni. J. Mater. Chem. C 2018, 6 (18), 4887−4894. (2) Park, J.; Hyun, B. G.; An, B. W.; Im, H.-G.; Park, Y.-G.; Jang, J.; Park, J.-U.; Bae, B.-S. Flexible Transparent Conductive Films with High Performance and Reliability Using Hybrid Structures of Continuous Metal Nanofiber Networks for Flexible Optoelectronics. ACS Appl. Mater. Interfaces 2017, 9 (24), 20299−20305. (3) Zhou, Y.; Azumi, R. Carbon Nanotube Based Transparent Conductive Films: Progress, Challenges, and Perspectives. Sci. Technol. Adv. Mater. 2016, 17 (1), 493−516. (4) Zhang, J.; Terrones, M.; Park, C. R.; Mukherjee, R.; Monthioux, M.; Koratkar, N.; Kim, Y. S.; Hurt, R.; Frackowiak, E.; Enoki, T.; et al. Carbon Science in 2016: Status, Challenges and Perspectives. Carbon 2016, 98, 708−732. (5) Chae, S. H.; Lee, Y. H. Carbon Nanotubes and Graphene towards Soft Electronics. Nano Converg. 2014, 1 (1), 15. (6) Jia, S.-L.; Geng, H.-Z.; Wang, L.; Tian, Y.; Xu, C.-X.; Shi, P.-P.; Gu, Z.-Z.; Yuan, X.-S.; Jing, L.-C.; Guo, Z.-Y.; et al. Carbon 3964

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The Journal of Physical Chemistry Letters Tunable Conductivity and Transparency by a Dry Transfer Technique. Nano Lett. 2010, 10 (11), 4349−4355. (27) Satco, D.; Nugraha, A. R. T.; Ukhtary, M. S.; Kopylova, D.; Nasibulin, A. G.; Saito, R. Intersubband Plasmon Excitations in Doped Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2019, 99 (7), No. 075403. (28) Shandakov, S. D.; Lomakin, M. V.; Nasibulin, A. G. The Effect of the Environment on the Electronic Properties of Single-Walled Carbon Nanotubes. Tech. Phys. Lett. 2016, 42 (11), 1071−1075. (29) Chegel, R. Third-Nearest-Neighbors Tight-Binding Description of Optical Response of Carbon Nanotubes: Effects of Chirality and Diameter. J. Electron. Mater. 2015, 44 (10), 3500−3511. (30) Harris, J. M.; Headrick, R. J.; Semler, M. R.; Fagan, J. A.; Pasquali, M.; Hobbie, E. K. Impact of SWCNT Processing on Nanotube-Silicon Heterojunctions. Nanoscale 2016, 8, 7969−7977. (31) Romanov, S. A.; Aliev, A. E.; Fine, B. V.; Anisimov, A. S.; Nasibulin, A. G. Highly Efficient Thermophones Based on Freestanding Single-Walled Carbon Nanotube Films. Nanoscale Horizons 2019, DOI: 10.1039/C9NH00164F. (32) Eustis, S.; El-Sayed, M. A. Why Gold Nanoparticles Are More Precious than Pretty Gold: Noble Metal Surface Plasmon Resonance and Its Enhancement of the Radiative and Nonradiative Properties of Nanocrystals of Different Shapes. Chem. Soc. Rev. 2006, 35 (3), 209− 217. (33) Gilshteyn, E. P.; Lin, S.; Kondrashov, V. A.; Kopylova, D. S.; Tsapenko, A. P.; Anisimov, A. S.; Hart, A. J.; Zhao, X.; Nasibulin, A. G. A One-Step Method of Hydrogel Modification by Single-Walled Carbon Nanotubes for Highly Stretchable and Transparent Electronics. ACS Appl. Mater. Interfaces 2018, 10 (33), 28069−28075. (34) Wang, X.; Liu, Z.; Zhang, T. Flexible Sensing Electronics for Wearable/Attachable Health Monitoring. Small 2017, 13, 1602790. (35) Zhang, Q.; Tan, L. F.; Chen, Y. X.; Zhang, T.; Wang, W. J.; Liu, Z.; Fu, L. Human-like Sensing and Reflexes of Graphene-Based Films. Adv. Sci. 2016, 3 (12), 1600130. (36) Duong, D. L.; Yun, S. J.; Lee, Y. H. Van Der Waals Layered Materials: Opportunities and Challenges. ACS Nano 2017, 11 (12), 11803−11830.

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