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Doping of active electrode materials for electrochemical batteries: an electronic structure perspective

Published online by Cambridge University Press:  14 August 2017

Johann Lüder
Affiliation:
Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore
Fleur Legrain
Affiliation:
Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore
Yingqian Chen
Affiliation:
Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore
Sergei Manzhos*
Affiliation:
Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore
*
Address all correspondence to Sergei Manzhos at mpemanzh@nus.edu.sg
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Abstract

Doping is a potent and often used strategy to modify properties of active electrode materials in advanced electrochemical batteries. There are several factors by which doping changes properties critically affecting battery performance, most notably the voltage, capacity, rate capability, and stability. These factors have to do specifically with changes in structure, band gap and band structure, and structural instability induced by doping. We review our recent modeling works on the effects of doping of active electrode materials, notably for prospective materials for organic and post-lithium (Na ion, Mg ion) batteries, as well as present new results, to build a coherent view on the use of n- and p-doping to modulate Li, Na, and Mg storage properties, most notably voltage. Specifically, we clearly point out effects due to electronic structure and those due to strain (structural instability), which clears some confusion about the effects of n- versus p-doping and facilitates rational rather than ad hoc design of doped materials.

Type
Prospective Articles
Copyright
Copyright © Materials Research Society 2017 

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Footnotes

Present address: CEA, LITEN, 17 Rue des Martyrs, 38054 Grenoble, France.

References

1.Kousksou, T., Bruel, P., Jamil, A., El Rhafiki, T., and Zeraouli, Y.: Energy storage: applications and challenges. Sol. Energy Mater. Sol. Cells 120 (Part A), 5980 (2014).Google Scholar
2.Manthiram, A.: Electrical energy storage: materials challenges and prospects. MRS Bull. 41, 624631 (2016).Google Scholar
3.Crabtree, G., Kócs, E., and Trahey, L.: The energy-storage frontier: lithium-ion batteries and beyond. MRS Bull. 40, 10671078 (2015).Google Scholar
4.Wang, Y., Chen, R., Chen, T., Lv, H., Zhu, G., Ma, L., Wang, C., Jin, Z., and Liu, J.: Emerging non-lithium ion batteries. Energy Storage Mater. 4, 103129 (2016).Google Scholar
5.Chen, H., Armand, M., Courty, M., Jiang, M., Grey, C.P., Dolhem, F., Tarascon, J.-M., and Poizot, P.: Lithium salt of tetrahydroxybenzoquinone: toward the development of a sustainable Li-ion battery. J. Am. Chem. Soc. 131, 89848988 (2009).Google Scholar
6.Larcher, D. and Tarascon, J.-M.: Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 7, 1929 (2014).Google Scholar
7.Sano, H., Senoh, H., Yao, M., Sakaebe, H., and Kiyobayashi, T.: Mg2 storage in organic positive-electrode active material based on 2,5-dimethoxy-1,4-benzoquinone. Chem. Lett. 41, 15941596 (2012).Google Scholar
8.Sawicki, M. and Shaw, L.L.: Advances and challenges of sodium ion batteries as post lithium ion batteries. RSC Adv. 5, 5312953154 (2015).Google Scholar
9.Levi, E., Levi, M.D., Chasid, O., and Aurbach, D.: A review on the problems of the solid state ions diffusion in cathodes for rechargeable Mg batteries. J. Electroceram. 22, 1319 (2009).Google Scholar
10.Islam, M.S. and Fisher, C.A.J.: Lithium and sodium battery cathode materials: computational insights into voltage, diffusion and nanostructural properties. Chem. Soc. Rev. 43, 185 (2014).Google Scholar
11.Stevens, D.A. and Dahn, J.R.: The mechanisms of lithium and sodium insertion in carbon materials. J. Electrochem. Soc. 148, A803A811 (2001).Google Scholar
12.Kulish, V. and Manzhos, S.: Comparison of Li, Na, Mg and Al-ion insertion in vanadium pentoxides and vanadium dioxides. RSC Adv. 7, 18643 (2017).Google Scholar
13.Ge, M., Rong, J., Fang, X., and Zhou, C.: Porous doped silicon nanowires for lithium ion battery anode with long cycle life. Nano Lett. 12, 23182323 (2012).Google Scholar
14.Komaba, S., Matsuura, Y., Ishikawa, T., Yabuuchi, N., Murata, W., and Kuze, S.: Redox reaction of Sn-polyacrylate electrodes in aprotic Na cell. Electrochem. Commun. 21, 6568 (2012).Google Scholar
15.Oltean, V.-A., Renault, S., Valvo, M., and Brandell, D.: Sustainable materials for sustainable energy storage: organic Na electrodes. Materials 9, 142 (2016).Google Scholar
16.Nisula, M. and Karppinen, M.: Atomic/molecular layer deposition of lithium terephthalate thin films as high rate capability Li-ion battery anodes. Nano Lett. 16, 12761281 (2016).Google Scholar
17.Park, Y., Shin, D.-S., Woo, S.H., Choi, N.S., Shin, K.H., Oh, S.M., Lee, K.T., and Hong, S.Y.: Sodium terephthalate as an organic anode material for sodium ion batteries. Adv. Mater. 24, 3562–3356 (2012).Google Scholar
18.Sk, M.A. and Manzhos, S.: Exploring the sodium storage mechanism in disodium terephthalate as anode for organic battery using density-functional theory calculations. J. Power Sources 324, 572581 (2016).Google Scholar
19.Zhao, L., Zhao, J., Hu, Y.-S., Li, H., Zhou, Z., Armand, M., and Chen, L.: Disodium terephthalate (Na2C8H4O4) as high performance anode material for low-cost room-temperature sodium-ion battery. Adv. Energy Mater. 2, 962965 (2012).Google Scholar
20.Wang, L., He, X., Sun, W., Li, J., Gao, J., Tian, G., Wang, J., and Fan, S.: Organic polymer material with a multi-electron process redox reaction: towards ultra-high reversible lithium storage capacity. RSC Adv. 3, 3227 (2013).Google Scholar
21.Yao, M., Senoh, H., Yamazaki, S.-I., Siroma, Z., Sakai, T., and Yasuda, K.: High-capacity organic positive-electrode material based on a benzoquinone derivative for use in rechargeable lithium batteries. J. Power Sources 195, 83368340 (2010).Google Scholar
22.Liu, T., Kim, K.C., Lee, B., Chen, Z., Noda, S., Jang, S.S., and Lee, S.W.: Self-polymerized dopamine as an organic cathode for Li- and Na-ion batteries. Energy Environ. Sci. 10, 205215 (2017).Google Scholar
23.Liang, Z., Tao, Z., and Chen, J.: Organic electrode materials for rechargeable lithium batteries. Adv. Energy Mater. 2, 742769 (2012).Google Scholar
24.Koshika, K., Sano, N., Oyaizu, K., and Nishide, H.: An aqueous, electrolyte-type, rechargeable device utilizing a hydrophilic radical polymer-cathode. Macromol. Chem. Phys. 210, 19891995 (2009).Google Scholar
25.Chen, Y. and Manzhos, S.: Lithium and sodium storage on tetracyanoethylene (TCNE) and TCNE-(doped)-graphene complexes: a computational study. Mater. Chem. Phys. 156, 180187 (2015).Google Scholar
26.Deng, Q., Fan, C., Wang, L., Cao, B., Jin, Y., Che, C.-M., and Li, J.: Organic potassium terephthalate (K2C8H4O4) with stable lattice structure exhibits excellent cyclic and rate capability in Li-ion batteries. Electrochim. Acta 222, 10861093 (2016).Google Scholar
27.Ratnakumar, B.V., Di Stefano, S., Williams, R.M., Nagasubramanian, G., and Bankston, C.P.: Organic cathode materials in sodium batteries. J. Appl. Electrochem. 20, 357364 (1990).Google Scholar
28.Slater, M.D., Kim, D., Lee, E., and Johnson, C.S.: Sodium-ion batteries. Adv. Funct. Mater. 23, 947958P (2012).Google Scholar
29.Serras, P., Palomares, V., Goni, A., Gil de Muro, I., Kubiak, P., Lezama, L., and Rojo, T.: High voltage cathode materials for Na-ion batteries of general formula Na3V2O2x(PO4)2F3−2x. J. Mater. Chem. 22, 2230122308 (2012).Google Scholar
30.Malyi, O.I., Tan, T.L., and Manzhos, S.: In search of high performance anode materials for Mg batteries: computational studies of Mg in Ge, Si, and Sn. J. Power Sources 233, 341345 (2013).Google Scholar
31.Sun, X., Bonnick, P., and Nazar, L.F.: Layered TiS2 positive electrode for Mg batteries. ACS Energy Lett. 1, 297301 (2016).Google Scholar
32.Wenzel, S., Hara, T., Janek, J., and Adelhelm, P.: Room-temperature sodium-ion batteries: improving the rate capability of carbon anode materials by templating strategies. Energy Environ. Sci. 4, 33423345 (2011).Google Scholar
33.Buiel, E. and Dahn, J.: Li-insertion in hard carbon anode materials for Li-ion batteries. Electrochim. Acta 45, 121130 (1999).Google Scholar
34.Saravanan, K., Mason, C.W., Rudola, A., Wong, K.H., and Balaya, P.: The first report on excellent cycling stability and superior rate capability of Na3V2(PO4)3 for sodium ion batteries. Adv. Energy Mater. 3, 444450 (2013).Google Scholar
35.Rudola, A., Sharma, N., and Balaya, P.: Introducing a 0.2 V sodium-ion battery anode: the Na2Ti3O7 to Na2Ti3−xO7 pathway. Electrochem. Commun. 61, 1013 (2015).Google Scholar
36.Rudola, A., Saravanan, K., Devaraj, S., Gong, H., and Balaya, P.: Na2Ti6O13: a potential anode for grid-storage sodium-ion batteries. Chem. Commun. 49, 74517453 (2013).Google Scholar
37.Hanyu, Y. and Honma, I.: Rechargeable quasi-solid state lithium battery with organic crystalline cathode. Sci. Rep. 2, 453 (2012).Google Scholar
38.Chen, Y. and Manzhos, S.: A computational study of lithium interaction with tetracyanoethylene (TCNE) and tetracyaniquinodimethane (TCNQ) molecules. Phys. Chem. Chem. Phys. 18, 14701477 (2016).Google Scholar
39.Chen, Y. and Manzhos, S.: Comparative computational study of lithium and sodium insertion in van der Waals and covalent tetracyanoethylene (TCNE)-based crystals as promising materials for organic lithium and sodium ion batteries. Phys. Chem. Chem. Phys. 18, 8874 (2016).Google Scholar
40.Chen, Y. and Manzhos, S.: Li storage on TCNE and TCNE-(doped)-graphene complexes: a computational study. MRS Proc. 1679 (2014). doi: 10.1557/opl.2014.849.Google Scholar
41.Chen, Y., Lüder, J., and Manzhos, S.: Disodium pyridine dicarboxylate vs disodium terephthalate as anode materials for organic Na ion batteries: effect of molecular structure on voltage from the molecular modeling perspective. MRS Adv. 15 (2017). doi: 10.1557/adv.2017.323.Google Scholar
42.Ortiz-Vitoriano, N., Drewett, N.E., Gonzalo, E., and Rojo, T.: High performance manganese-based layered oxide cathodes: overcoming the challenges of sodium ion batteries. Energy Environ. Sci. 10, 10511074 (2017).Google Scholar
43.Kim, H., Ida, S., Ju, Y.-W., Matsuda, J., Kim, G., and Ishihara, T.: Mixing effects of Cr2O3-PrBaMn2O5 for increased redox cycling properties of Fe powder for a solid-oxide Fe-air rechargeable battery. J. Mater. Chem. A 5, 364371 (2017).Google Scholar
44.Thiele, P., Neumann, J., Westphal, A., Ludwig, R., Bonsa, A.-M., Appelhagen, A., Malcher, P., and Köckerling, M.: Electrical energy storage by a magnesium-copper-sulfide rechargeable battery. J. Electrochem. Soc. 164, A770A774 (2017).Google Scholar
45.Shi, B., Liu, W., Zhu, K., and Xie, J.: Synthesis of flower-like copper sulfides microspheres as electrode materials for sodium secondary batteries. Chem. Phys. Lett. 677, 7074 (2017).Google Scholar
46.Mohanty, S.P. and Nookala, M.: Investigation of in situ grown and carbon-free copper sulfide electrode for rechargeable lithium battery. J. Electroanal. Chem. 794, 814 (2017).Google Scholar
47.Liu, X., Huang, J.-Q., Zhang, Q., and Mai, L.: Nanostructured metal oxides and sulfides for lithium–sulfur batteries. Adv. Mater. 29, 1601759 (2017).Google Scholar
48.Song, S., Kotobuki, M., Chen, Y., Manzhos, S., Xu, C., Hu, N., and Lu, L.: Na-rich layered Na2Ti1−xCrxO3−x/2 (x = 0, 0.06): Na-ion battery cathode materials with high capacity and long cycle life. Sci. Rep. 7, 373 (2017).Google Scholar
49.Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., and Taga, Y.: Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 293, 269271 (2001).Google Scholar
50.Aman, N., Das, N.N., and Mishra, T.: Effect of N-doping on visible light activity of TiO2–SiO2 mixed oxide photocatalysts. J. Environ. Chem. Eng. 4, 191196 (2016).Google Scholar
51.Schneider, T., Limberg, F., Yao, K., Armin, A., Jürgensen, N., Czolk, J., Ebenhoch, B., Friederich, P., Wenzel, W., Behrends, J., Krüger, H., and Colsmann, A.: p-Doping of polystyrene polymers with attached functional side-groups from solution. J. Mater. Chem. C 5, 770776 (2017).Google Scholar
52.Pingel, P., Schwarzl, R., and Neher, D.: Effect of molecular p-doping on hole density and mobility in poly(3-hexylthiophene). Appl. Phys. Lett. 100, 143303 (2012).Google Scholar
53.Goodenough, J.B.: Rechargeable batteries: challenges old and new. J. Solid State Electrochem. 16, 20192029 (2012).Google Scholar
54.Li, W., Song, B., and Manthiram, A.: High-voltage positive electrode materials for lithium-ion batteries. Chem. Soc. Rev. 46, 30063059 (2017).Google Scholar
55.Molenda, J., Baster, D., Molenda, M., Swierczek, K., and Tobola, J.: Anomaly in the electronic structure of the NaxCoO2−y cathode as a source of its step-like discharge curve. Phys. Chem. Chem. Phys. 16, 1484514857 (2014).Google Scholar
56.Lüder, J., Cheow, M.H., and Manzhos, S.: Understanding doping strategies in the design of organic electrode materials for Li and Na ion batteries: an electronic structure perspective. Phys. Chem. Chem. Phys. 19, 1319513209 (2017).Google Scholar
57.Ceder, G., Aydinol, M., and Kohan, A.: Application of first-principles calculations to the design of rechargeable Li-batteries. Comput. Mater. Sci. 8, 161169 (1997).Google Scholar
58.Legrain, F. and Manzhos, S.: Aluminum doping improves the energetics of lithium, sodium, and magnesium storage in silicon: a first-principles study. J. Power Sources 274, 6570 (2015).Google Scholar
59.Hirai, K., Ichitsubo, T., Uda, T., Miyazaki, A., Yagi, S., and Matsubara, E.: Effects of volume strain due to Li–Sn compound formation on electrode potential in lithium-ion batteries. Acta Mater. 56, 15391545 (2008).Google Scholar
60.Legrain, F. and Manzhos, S.: A first-principles comparative study of lithium, sodium, and magnesium storage in pure and gallium-doped germanium: competition between interstitial and substitutional sites. J. Chem. Phys. 146, 034706 (2017).Google Scholar
61.Legrain, F., Malyi, O.I., and Manzhos, S.: Comparative computational study of the diffusion of Li, Na, and Mg in silicon including the effect of vibrations. Solid State Ion. 253, 157163 (2013).Google Scholar
62.Huang, Y., Wu, D., Dianat, A., Bobeth, M., Huang, T., Mai, Y., Zhang, F., Cuniberti, G., and Feng, X.: Bipolar nitrogen-doped graphene frameworks as high-performance cathodes for lithium ion batteries. J. Mater. Chem. A 5, 15881594 (2017).Google Scholar
63.Wehling, T.O., Novoselov, K.S., Morozov, S.V., Vdovin, E.E., Katsnelson, M.I., Geim, A.K., and Lichtenstein, A.I.: Molecular doping of graphene. Nano Lett. 8, 173177 (2008).Google Scholar
64.Morita, S., Zakhidov, A.A., and Yoshino, K.: Doping effect of buckminsterfullerene in conducting polymer: change of absorption spectrum and quenching of luminescene. Solid State Commun. 82, 249252 (1992).Google Scholar
65.Chen, Y. and Manzhos, S.: Voltage and capacity control of polyaniline based organic cathodes: an ab initio study. J. Power Sources 336, 126131 (2016).Google Scholar
66.Hohenberg, P. and Kohn, W.: Inhomogeneous electron gas. Phys. Rev. 136, B864B871 (1964).Google Scholar
67.Kohn, W. and Sham, L.J.: Self-consistent equations including exchange and correlation effects. Phys. Rev. 140, A1133A1138 (1965).Google Scholar
68.Aydinol, M.K., Kohan, A.F., Ceder, G., Cho, K., and Joannopoulos, J.: Ab initio study of lithium intercalation in metal oxides and metal dichalcogenides. Phys. Rev. B 56, 13541365 (1997).Google Scholar
69.Urban, A., Seo, D.-H., and Ceder, G.: Computational understanding of Li-ion batteries. NPJ Comput. Mater. 2, 16002 (2016).Google Scholar
70.Zhou, F., Cococcioni, M., Marianetti, C.A., Morgan, D., and Ceder, G.: First-principles prediction of redox potentials in transition-metal compounds with LDA + U. Phys. Rev. B 70, 235121 (2004).Google Scholar
71.Legrain, F., Malyi, O.I., Persson, C., and Manzhos, S.: Comparison of alpha and beta tin for lithium, sodium, and magnesium storage: an ab initio study including phonon contributions. J. Chem. Phys. 143, 204701 (2015).Google Scholar
72.Rodriguez-Perez, I.A., Jian, Z., Waldenmaier, P.K., Palmisano, J.W., Chandrabose, R.S., Wang, X., Lerner, M.M., Carter, R.G., and Ji, X.: A hydrocarbon cathode for dual-ion batteries. ACS Energy Lett. 1, 719723 (2016).Google Scholar
73.Yasuda, T. and Ogihara, N.: Reformation of organic dicarboxylate electrode materials for rechargeable batteries by molecular self-assembly. Chem. Commun. 50, 1156511567 (2014).Google Scholar
74.Ogihara, N., Yasuda, T., Kishida, Y., Ohsuna, T., Miyamoto, K., and Ohba, N.: Organic dicarboxylate negative electrode materials with remarkably small strain for high-voltage bipolar batteries. Angew. Chem. 53, 1146711472 (2014).Google Scholar
75.Armand, M., Grugeon, S., Vezin, H., Laruelle, S., Ribière, P., Poizot, P., and Tarascon, J.-M.: Conjugated dicarboxylate anodes for Li-ion batteries nature materials. Nature 8, 120125 (2009).Google Scholar
76.Chakrapani, V.: Electrochemical transfer doping: a novel phenomenon seen in diamond, gallium nitride, and carbon nanotubes. ECS Trans. 66, 2937 (2015).Google Scholar
77.Reddy, A.L.M., Srivastava, A., Gowda, S.R., Gullapalli, H., Dubey, M., and Ajayan, P.M.: Synthesis of nitrogen-doped graphene films for lithium battery application. ACS Nano 4, 63376342 (2010).Google Scholar
78.Padhy, H., Chen, Y., Lüder, J., Reddy, G.S., Manzhos, S., and Balaya, P.: Charge and discharge processes and sodium storage in disodium pyridine-2,5-dicarboxylate anode—insights from experiments and theory. Submitted to Advanced Functional Materials.Google Scholar
79.Zhang, L., Hu, X., Chen, C., Guo, H., Liu, X., Xu, G., Zhong, H., Cheng, S., Wu, P., Meng, J., Huang, Y., Dou, S., and Liu, H.: In operando mechanism analysis on nanocrystalline silicon anode material for reversible and ultrafast sodium storage. Adv. Mater. 29, 1604708 (2017).Google Scholar
80.Li, J.-Y., Xu, Q., Li, G., Yin, Y.-X., Wan, L.-J., and Guo, Y.-G.: Research progress regarding Si-based anode materials towards practical application in high energy density Li-ion batteries. Mater. Chem. Front. (2017). doi: 10.1039/C6QM00302H.Google Scholar
81.Baggetto, L., Keum, J.K., Browning, J.F., and Veith, G.M.: Germanium as negative electrode material for sodium-ion batteries. Electrochem. Commun. 34, 4144 (2013).Google Scholar
82.Malyi, O.I., Tan, T.L., and Manzhos, S.: A comparative computational study of structures, diffusion, and dopant interactions between Li and Na insertion into Si. Appl. Phys. Express 6, 027301 (2013).Google Scholar
83.Kohandehghan, A., Cui, K., Kupsta, M., Ding, J., Lotfabad, E.M., Kalisvaart, W.P., and Mitlin, D.: Activation with Li enables facile sodium storage in germanium. Nano Lett. 14, 58735882 (2014).Google Scholar
84.Legrain, F., Malyi, O.I., and Manzhos, S.: Comparative computational study of the energetics of Li, Na, and Mg storage in amorphous and crystalline silicon. Comput. Mater. Sci. 94, 214217 (2014).Google Scholar
85.McDowell, M.T., Lee, S.W., Nix, W.D., and Cui, Y.: 25th Anniversary article: understanding the lithiation of silicon and other alloying anodes for lithium-ion batteries. Adv. Mater. 25, 49664985 (2013).Google Scholar
86.Liu, X.H., Zhang, L.Q., Zhong, L., Liu, Y., Zheng, H., Wang, J.W., Cho, J.-H., Dayeh, S.A., Picraux, S.T., Sullivan, J.P., Mao, S.X., Ye, Z.Z., and Huang, J.Y.: Ultrafast electrochemical lithiation of individual Si nanowire anodes. Nano Lett. 11, 22512258 (2011).Google Scholar
87.Jin, W., Li, Z., Wang, Z., and Fu, Y.: Mg ion dynamics in anode materials of Sn and Bi for Mg-ion batteries. Mater. Chem. and Phys. 182, 167172 (2016).Google Scholar
88.Nguyen, D.-T., Tran, X.M., Kang, J., and Song, S.-W.: Magnesium storage performance and surface film formation behavior of tin anode material. ChemElectroChem 3, 18131819 (2016).Google Scholar
89.Legrain, F., Malyi, O.I., and Manzhos, S.: A comparative computational study of Li, Na, and Mg insertion in α-Sn. MRS Proc. 1678 (2014). doi: 10.1557/opl.2014.743.Google Scholar
90.Singh, N., Arthur, T.S., Ling, C., Matsui, M., and Mizuno, F.: A high energy-density tin anode for rechargeable magnesium-ion batteries. Chem. Commun. 49, 149151 (2013).Google Scholar
91.Heeger, A.J.: Semiconducting and metallic polymers: the fourth generation of polymeric materials. J. Phys. Chem. B 105, 84758491 (2001).Google Scholar
92.Zhao, Q., Lu, Y., and Chen, J.: Advanced organic electrode materials for rechargeable sodium-ion batteries. Adv. Energy Mater. 7, 1601792 (2017).Google Scholar
93.Oyaizu, K., Choi, W., and Nishide, H.: Functionalization of poly(4-chloromethylstyrene) with anthraquinone pendants for organic anode-active materials. Polym. Adv. Tech. 22, 12421247 (2011).Google Scholar
94.Pelzer, K., Cheng, L., and Curtiss, L.A.: Effects of functional groups in redox-active organic molecules: a high-throughput screening approach. J. Phys. Chem. C 121, 237245 (2017).Google Scholar
95.Coletti, C., Riedl, C., Lee, D.S., Krauss, B., Patthey, L., von Klitzing, K., Smet, J.H., and Starke, U.: Charge neutrality and band-gap tuning of epitaxial graphene on SiC by molecular doping. Phys. Rev. B 81, 235401 (2010).Google Scholar
96.Shim, M., Javey, A., Shi Kam, N.W., and Dai, H.: Polymer functionalization for air-stable n-type carbon nanotube field-effect transistors. J. Am. Chem. Soc. 123, 1151211513 (2001).Google Scholar
97.Masaoka, S., Mukawa, Y., and Sakai, K.: Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H2-evolving activity and the energy level of the LUMO. Dalton Trans. 39, 58685876 (2010).Google Scholar