Fine Art of Thermoelectricity - ACS Applied Materials & Interfaces

Jan 16, 2018 - On the other hand, conductive conjugated polymers(9) and organic–inorganic hybrid nanocomposites, such as polymer/carbon nanotubes,(1...
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Fine Art of Thermoelectricity Viktor Brus, Marc A. Gluba, Joerg Rappich, Felix Lang, Zakhar Kovalyuk, Pavlo Maryanchuk, and Norbert H. Nickel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17491 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Fine Art of Thermoelectricity Viktor V. Brus1*, Marc Gluba1, Jörg Rappich1, Felix Lang1, Pavlo. D. Maryanchuk2, Norbert H. Nickel1 1

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium Photovoltaik, Berlin, 12489, Germany 2 Chernivtsi National University, Department of Electronics and Energy Engineering, Kotsubynskiy 2, 58002 Chernivtsi, Ukraine

Abstract A detailed study of hitherto unknown electrical and thermoelectric properties of graphite pencil traces on paper was carried out by measuring the Hall and Seebeck effects. We show that the combination of pencil-drawn graphite and brush-painted PEDOT:PSS films on regular office paper results in extremely simple, low-cost and environmentally friendly thermoelectric power generators with promising output characteristics at low-temperature gradients. The working characteristics can be improved even further by incorporating n-type InSe flakes. The combination of pencil drawn n-InSe:graphite nanocomposites and brush-painted PEDOT:PSS increases the power output by one order of magnitude. Keywords: pencil trace, PEDOT:PSS, paper, thermoelectricity, InSe

Introduction Natural and artificial energy sources dissipate heat that is lost to the surrounding environment. Typically, the associated temperature-gradients are too small to operate steam turbines.1-3 In contrast, thermoelectric elements enable a direct transformation of waste heat to electric power at very small temperature differences up to few degrees.4,

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Therefore,

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thermoelectric power generation is considered as one of the most important alternative energy sources.6 So far, the thermoelectric community is focused on bulk inorganic semiconductors, such as metal chalcogenides, for example, Bi2Te3-based materials which are used in the most efficient and stable thermoelectric devices.7, 8 These conventional thermoelectric materials are expensive, energy consuming in preparation and environmentally hazardous. On the other hand, conductive conjugated polymers9 and organic-inorganic hybrid nanocomposites such as polymer/carbon nanotubes10 turn out to be very promising materials that can be implemented in large-scale, lowcost, and flexible thermoelectric elements based on thin film architectures. Flexible thin film organic thermoelectric generators open new possibilities for the development of miniature mobile and remote electronic gadgets like intelligent wireless sensors and electronic medical implants. As usually organic semiconductors with promising thermoelectric properties exhibit ptype of conductivity, for instance, PEDOT:PSS.11-14 Therefore, the development of new low-cost n-type thermoelectric thin film materials on flexible substrates is of a great practical importance. Surprisingly, an interesting approach originates from the simple drawing with a graphite pencil on regular office paper. Firstly, paper is cheap and decomposable. Secondly, paper is an insulating material and has been successfully used in electrical engineering for decades. Nowadays paper is often considered as a flexible substrate due to recent advances in printed and drawn electronics.15, 16 Despite the low electrical and thermal conductivity of paper, there are still only few reports on paper-based thin-film thermoelectric elements.17, 18 The lead in a pencil is usually made of graphite powder and inorganic clay in different ratios. Pencil traces consist of a nanocomposite that contains graphite micro- and nanoparticles, multi-layer nano-sheets of

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graphene, and clay.19 The traces are electrically conductive and stable in different environments. Pencil-drawn thin-films have already been implemented in different novel electronic and sensor devices such as super capacitors20, terahertz generators21, field-effect transistors (FETs)22, pencilon-semiconductor photodiodes23, 24, as well as photo-, tenzo- and, chemi-resistive sensors.25-29 Recently, we have proposed a novel energy harvesting ionic-organic electronic ratchet with pencil drawn electrodes that is capable of converting zero time-average electromagnetic noise signals within a wide range of frequencies into an useful DC electric current for the operation of remote low-power electronics or medical implants.30 In this work we consider the thermoelectric properties of graphite and n-InSe:graphite pencil-traces. Moreover, we present a new concept of pencil trace/PEDOT:PSS thermoelectric power generators, drawn and painted on regular office paper.

Results and discussions Office paper was used as the substrate material (Fig. 1a and 1b). The graphite films were drawn by a commercial HB pencil. The PEDOT:PSS films were painted by a paintbrush. Scanning electron microscopy (SEM) and Raman backscattering measurements were employed to inspect the morphology and structural properties of the pencil-drawn traces and PEDOT:PSS films on paper. The pencil traces on paper consist of disordered stacks of graphite flakes with different sizes (see Fig. 1c and 1d). The G and 2D modes of the Raman spectrum of the pencil trace correlate with the Raman spectrum of highly oriented pyrolytic graphite (HOPG), considered as a reference. However, the pronounced D mode originates from the disordered structure of the pencil drawn graphite film.31 The PEDOT-PSS water suspension penetrates into the paper substrate by filling voids between cellulose fibers (Fig. 1b) and thus, the morphology

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of the painted PEDOT-PSS film partially repeats the morphology of the bare paper surface (Fig. 1f and 1g). The Raman spectrum of the PEDOT:PSS film on paper (Fig. 1h), exhibits the characteristic phonon modes of the PEDOT:PSS polymer32, 33 since paper is a Raman inactive material (see inset in Fig. 1h).

Figure 1.a) – graphite pencil and PEDOT:PSS traces on regular paper; b) – morphology of the paper surface; c) and d) – morphology of the pencil trance on paper; e) – Raman backscattering spectra of the pencil trace and high oriented pyrolytic graphite as a reference; f) and g) – morphology of the PEDOT:PSS trace on paper; h) – Raman backscattering spectrum of the PEDOT:PSS trace on paper. The inset shows the Raman backscattering spectrum of the bare paper.

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The electrical properties of the graphite and the PEDOT:PSS films on paper were investigated by Hall-effect measurements using the van-der-Pauw geometry. The measured values of the specific electrical conductivity σ, the Hall coefficient RH, the Hall mobility µ, and the density of charge carriers n, are summarized in Table 1 and compared to those of HOPG. Table 1. Electrical properties of the samples under investigation. Sample

µ (cm2/Vs)

σ

RH

(Ohm-1cm-1)

(cm3/C)

HOPG

3.5×103

-3.89×10-1

1362

-1.6×1019

Graphite

5.8×102

-4.14×10-3

2.44

-1.51×1021

1.87×102

-2.8×10-3

0.53

-2.2×1021

1.18

-6.87×10-1

0.67

-1.1×1019

n (cm-3)

trace Pencil (HB) trace PEDOT:PSS

In case of the HOPG sample and the pencil-drawn film a negative Hall coefficient RH is obtained reflecting electron conductance. The low value of the Hall mobility in the drawn graphite film are caused by its disordered structure (Fig. 1d and 1c). The PEDOT:PSS film also exhibits a negative value of RH, which, however, is due to hopping transport of holes in PEDOT:PSS.34 The negative Hall coefficient in p-type PEDOT:PSS films has already been independently reported.35 The p-type conductivity of the PEDOT:PSS film on paper will be confirmed by its positive Seebeck coefficient, considered below. The further analysis of the type of conductivity and thermoelectric properties of the drawn graphite and PEDOT:PSS films on paper was carried out by measuring the Seebeck

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coefficient S, which describes fundamental electronic transport properties of thermoelectric materials. A schematic picture of the Seebeck effect measurement in this study is shown in Fig. 2.

Figure 2. Schematic representation of the Seebeck effect measurement. The experimental set-up consists of two water-cooled Peltier elements with a temperature controlled power source.

The Seebeck coefficient is determined by linear fitting of experimentally measured thermoelectric voltage ∆Vt at different temperature differences across the samples S = ∆Vt/∆T (experimental details are given in the experimental section). Fig. 3a and 3b show that S is negative for the graphite film and positive for the PEDOT:PSS film that indicates the electron and hole type of conductivity, respectively. It is worth noting that the measured value of the Seebeck coefficient of the pencil-drawn film on paper SG is comparable to that of other expensive electron conducting nanocomposite materials used in flexible thermoelectric devices (21 µV K-1)10. Thus, it is conceivable that the combination of pencil traces and PEDOT:PSS films

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on electrically and thermally insulating paper substrate can be used for a novel inexpensive and flexible thermoelectric device architecture.

Figure 3. Thermoelectric voltage as a function of temperature difference for a) pencil-drawn and b) PEDOT:PSS traces on paper. Further the determined Seebeck coefficients SG and SP are given for the graphite and PEDOT:PSS films, respectively.

The concept of a pencil-trace/PEDOT:PSS thermoelectric power generator is first realized by painting PEDOT:PSS layers over pencil traces on paper. Current-voltage (I-V) characteristics of the drawn single junction thermoelectric power generators were measuring as a function of the temperature difference between the junction and the edges of the devices, while the junction area

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was always kept at higher temperature. I-V characteristics of the single junction thermoelectric element measured at different temperature differences are shown in Fig. 4a.

Figure 4. The single junction thermoelectric element. a) I-V characteristics vs. temperature difference. The inset shows a picture of one of the measured single junction devices with the indication of the low T1 and high T2 temperature sides. b) – output power vs. voltage at different

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temperature gradients; c) – open circuit voltage and short circuit current vs. temperature difference.

As can be seen from Fig. 4b and 4c the main electrical characteristics: output power P, shortcircuit current Isc and open-circuit voltage Voc increase with an increase of the temperature difference between the edges and the junction of the thermoelectric element. It should be mentioned that an increase of the output power can be accomplished also by making thermoelectric batteries. Two thermoelectric elements, for example almost double the output power as demonstrated in Supporting Information Fig. S1. Fig. 4 and S1 exhibit that pencil-drawn graphite films are capable of working in flexible low-cost thermoelectric devices. However, the thermoelectric parameters of graphite traces are still quite low and should be further improved. This can be realized by mixing graphite and layered semiconductors with n-type of conductivity like InSe.36-38 Due to its layered structure InSe can be drawn on paper in the same manner as a graphite pencil. Recently, we have shown that the drawn nanocomposite film of graphite:layered semiconductor (GaSe) possesses a reasonable combination of the charge transport and photo-conductance properties.25 A similar approach may help to improve also the thermoelectrical properties of the drawn graphite films. A pencil lead was prepared by pressing a mixture of graphite and n-InSe powders in the mass ratio 1:1. The nanocomposite films n-InSe:graphite were also drawn on regular office paper. The Raman spectra of the InSe single crystal and the drawn n-InSe:graphite film is shown in Fig. 5.

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Figure 5. The comparison of the Raman spectra of the InSe single crystal and the nInSe:graphite drawn film on paper.

The peaks at 115, 178 and 227 cm-1 correspond to the A11g, E12g and A21g modes of the single crystal InSe.39,

40

The same peaks appear at the Raman spectra of the n-InSe:graphite film

together with the signals from the D, G and 2D modes of graphite. The drawn nanocomposite n-InSe:graphite films reveal a great improvement in the electron mobility and the Seebeck coefficient in comparison to that of the pure graphite film. However, the incorporation of InSe flakes into the drawn graphite film significantly reduces the electrical conductivity (see Table 2). The very low electrical conductivity of pure InSe films drawn on paper results in the negligibly low performance of thermoelectric devices based on them. Therefore, they were not considered in this study.

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Table 2. Electrical and thermoelectrical properties of pencil and InSe:Graphite traces on paper. Sample

σ

RH

(Ohm-1cm-1) (cm3/C) Pencil

µ

n

S

σS2

(cm2/Vs)

(cm-3)

(µV K-1)

(µWm-1K-2)

1.87×102

-2.8×10-3

0.53

-2.2×1021

-17.9

6

16.1

-1.74×10-1

28.12

-3.59×1018

-243

95

(HB) InSe:G

Fig. 6 shows the load I-V and output power characteristics of a single junction nInSe:graphite/PEDOT:PSS thermoelectric device fully drawn on paper.

Fig. 6. Output characteristics of the n-InSe:graphite/PEDOT:PSS thermoelectric generator at the temperature difference of 50 K.

The short circuit current slightly decreased, however, the open-circuit voltage significantly increased in comparison to graphite pencil traces. Consequently, the output power is increased by an order of magnitude due to the incorporation of the n-InSe flakes into the drawn graphite film, which served as the n-type film of the fully drawn hybrid organic-inorganic n-

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InSe:graphite/PEDOT:PSS thermoelectric generator on paper. This improvement reveals a great potential of the proposed concept of the drawn thermoelectric devices for further development and wide practical applications.

Conclusions In conclusion, we have studied the current transport and the thermoelectric properties of graphite pencil traces on paper. The Seebeck coefficient of graphite pencil-drawn films was negative and with a reasonably high value of SG = -17.5 µV/K comparable to that of other expensive electron conducting nanocomposite materials used in flexible thermoelectric devices (21 µV K-1)10. This allowed to draw graphite pencil trace/PEDOT:PSS thermoelectric elements on regular office paper. The fabrication of multi-junction thermoelectric power generators can be realized by drawing arrays of interconnected graphite pencil trace/PEDOT:PSS thermoelectric elements. Moreover, the n-InSe:graphite drawn nanocomposite films exhibit a significantly higher Seebeck coefficient up to -243 µV/K in comparison to the graphite pencil traces. Consequently, the output power of the n-InSe:Graphite/PEDOT:PSS thermoelectric devices was increased by one order of magnitude to over 10 nW at the temperature difference of 50 K that is comparable with optimized printed silver paint/Tellurium-PEDOT:PSS flexible thermoelectric generators.14 The hitherto unstudied thermoelectric properties of graphite and nanocomposite nInSe:graphite pencil traces on paper provide new opportunities for the development of flexible, extremely simple and cheap energy harvesters, that can be used for low-temperature waste heat sources. Our results convincingly exhibit proof-of-the concept thermoelectric devices drawn and painted on regular office paper.

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Experimental part Regular office paper with the density of 80 g m-2 was used as the substrate for all samples prepared in this study. The graphite films were drawn by a HB pencil lead with the diameter of 1 mm. The films were drawn while paper was mounted on an electronic balance. This simple approach allows controlling the applied vertical force of 1 N. The PEDOT:PSS films were painted with a paintbrush. On average 20 µl of a commercially available PEDOT:PSS water suspension (Heraeus PH 1000) was used per square centimeter. The PEDOT:PSS layers were painted over pencil traces to prepare the thermoelectric elements. The graphite pencil drawn and PEDOT:PSS films were 3 cm long and 5 mm wide. The painted PEDOT:PSS films were dried at room temperature and then annealed in air at 100 ˚C for 15 min. No additional coatings were used in this study. The area of the pencil trace/PEDOT:PSS junction was equal to ~25 mm2. InSe crystals were grown by the Bridgman technique. The crystals possessed n-type of conductivity with the electron concentration about 1016 cm-3. InSe crystals were mechanically grinded until the particles size was below 100 µm. The prepared InSe powder was mechanically mixed with a commercially available, high purity graphite powder (particles size ≤ 50 µm) for 20 min. The obtained InSe:Graphite powder was then pressed under a pressure of 5 tons/cm2 to form the composite pencil lead. Binding additives were not used in this study. The morphology and structural properties of the graphite, n-InSe:Grpahite and PEDOT:PSS films were examined using a scanning electron microscope (Hitachi S 4100) and a micro Raman spectrometer (LabRAM) with an excitation wavelength of 632.82 nm. The thickness of the graphite, n-InSe:Graphite and PEDOT:PSS films was estimated by means of a profilometer to be around 100 nm and 1.2 µm, respectively with relative deviations ±10%.

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For Hall-effect measurements the van-der-Pauw contact geometry was used. The Halleffect measurements were carried out in air and at room temperature.41 Silver paint was taken as a contact material. Gold electrodes were deposited onto graphite, n-InSe:Graphite and PEDOT:PSS films by thermal evaporation in a vacuum chamber for the measurements of the thermoelectric properties. The Seebeck coefficient S was determined by linear fitting of experimentally measured ∆Vt at different temperature difference between the contacts S = ∆Vt/∆T. The temperature difference was controlled by two thermocouples attached to the sample. Thermal paste was used to achieve good thermal contacts of the samples. The thermo-power setup was calibrated by measuring silicon and indium-tin-oxide samples with known Seebeck coefficients.

Corresponding Author *Corresponding Author: Dr. Viktor Brus e-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Supporting Information Output characteristics of two thermoelectric elements connected in series.

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Funding Sources V.V. Brus acknowledges the Alexander-von-Humboldt Foundation for financial support in the framework of the Georg Forster Research Fellowship.

ACKNOWLEDGMENT The authors are grateful to C. Klimm for taking SEM micrographs.

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(6) Liu, W.; Lie, Q.; Kim, H. S.; Ren, Z. Current progress and future challenges in thermoelectric power generation: From materials to devices Acta Materialia 2015, 87, 357376. (7) Han, Ch.; Sun, Q.; Li, Z.; Dou, S. X. Thermoelectric Enhancement of Different Kinds of Metal Chalcogenides Adv. Energy Mater. 2016, 6, 1600498. (8) Luo, Y.; Yang, J.; Jiang, Q.; Li, W.; Zhang, D.; Zhou, Z.; Cheng, Y.; Ren, Y.; He, X. Progressive Regulation of Electrical and Thermal Transport Properties to High-Performance CuInTe2 Thermoelectric Materials Adv. Energy Mater. 2016, 6, 1600007. (9) Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. A. Organic thermoelectric materials for energy harvesting and temperature control Nature Rev. Mater. 2016, 1, 16050. (10) Mai, Ch.-K.; Russ, B.; Fronk, S. L.; Hu, N.; Chan-Park, M. B.; Urban, J. J.; Segalman, R. A.; Chabinyc, M. L.; Bazan, G. C. Varying the ionic functionalities of conjugated polyelectrolytes leads to both p- and n-type carbon nanotube composites for flexible thermoelectrics Energy Environ. Sci. 2015, 8, 2341-2346. (11) Yue, R.; Xu, J. Poly(3,4-ethylenedioxythiophene) as promising organic thermoelectric materials: A mini-review Synthetic Metals 2012, 162, 912-917. (12) Wei, K.; Stedman, T.; Ge, Z.-H.; Woods, L. M.; Nolas, G. S. A synthetic approach for enhanced thermoelectric properties of PEDOT:PSS bulk composites Appl. Phys. Lett. 2015, 107, 153301. (13) Fan, Z.; Du, D.; Yu, Z.; Li, P.; Xia, Y.; Ouyang, J. Significant Enhancement in the Thermoelectric Properties of PEDOT:PSS Films through a Treatment with Organic Solutions of Inorganic Salts ACS Appl. Mater. Interfaces. 2016, 8, 23204-23211.

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(14) Bae, E. J.; Kang, Y. H.; Jang, K.-S.; Cho, S. Y. Enhancement of Thermoelectric Properties of PEDOT:PSS and Tellurium-PEDOT:PSS Hybrid Composites by Simple Chemical Treatment Scientific Reports 2016, 6, 18805. (15) Tobjork, D.; Osterbacka, R. Paper Electronics Adv. Mater. 2011, 23, 1935-1961. (16) Li, Z.; Liu, H.; Ouyang, Ch.; Wee, W. H.; Cui, X.; Lu, T. J.; Pingguan-Murphy, B.; Li, F.; Xu, F. Recent Advances in Pen-Based Writing Electronics and their Emerging Applications Adv. Funct. Mater. 2016, 26, 165-180. (17) Jiang, Q.; Liu, C.; Xu, J.; Lu. B.; Song, H.; Shi, H.; Yao. Y.; Zhang, L. Paper: An Effective Substrate for the Enhancement of Thermoelectric Properties in PEDOT:PSS J. Polymer Sci. Part B: Polymer Physics 2014, 52, 737-742. (18) Wei, Q.; Mukaida, M.; Kirihara, K.; Naitoh, Y.; Ishida, T. Polymer thermoelectric modules screen-printed on paper RSC Adv. 2014, 4, 28802-28806 (19) Wang, Y.; Zhou, H. To draw an air electrode of a Li–air battery by pencil Energy Environ. Sci. 2011, 4, 1704-1707. (20) Zheng, G.; Hu, L.; Wu, H.; Xie, X.; Cui, Y. Paper supercapacitors by a solvent-free drawing method Energy Environm Sci. 2011, 4, 3368-3373. (21) Ramakrishman, G.; Chakkittakandy, R.; Planken, P.C.M. Terahertz generation from graphite Optics Express 2009, 17, 16092-16099. (22) Kurra, N.; Kulkarni, G. U. Pencil-on-paper: electronic devices Lab Chip 2013, 13, 2866-2873. (23) Brus, V. V.; Maryanchuk, P. D. Photosensitive Shottky-type heterojunctions prepared by the drawing of graphite films Appl. Phys. Lett. 2014, 104, 173501.

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(24) Brus, V. V.; Maryanchuk, P. D. Graphite trace on water surface: a step toward the lowcost pencil-on-semiconductor electronics and optoelectronics Carbon 2014, 78, 613-616. (25) Brus, V. V.; Marynchuk, P. D.; Kovalyuk, Z. D.; Abashyn, S. L. 2D nanocomposite photoconductive sensors fully dry drawn on regular paper Nanotechnology 2015, 26, 255501. (26) Kang, T.-K. Tunable piezoresistive sensors based on pencil-on-paper Appl. Phys. Lett. 2014, 104, 073117. (27) Lin, Ch.-W.; Zhao, Z.; Kim, J.; Huang, J. Pencil Drawn Strain Gauges and Chemiresistors on Paper Sci. Reports 2014, 4, 3812. (28) Frazier, K. M.; Mirica, K. A.; Walish, J. J.; Swager, T. M. Fully-drawn carbon-based chemical sensors on organic and inorganic surfaces Lab Chip 2014, 14, 4059-4066. (29) Smith, M. K.; Jensen, K. E.; Pivak, P. A.; Mirica, K. A. Direct Self-Assembly of Conductive Nanorods of Metal–Organic Frameworks into Chemiresistive Devices on Shrinkable Polymer Films Chem. Mater. 2016, 28, 5264-5268. (30) Brus, V. V.; Collins, S. D.; Mikhnenko, O. V.; Wang, M.; Bazan, G. C.; Nguyen, T.-Q. Fabricating Low-Cost Ionic-Organic Electronic Ratchets with Graphite Pencil and Adhesive Tape Adv. Electron. Mater. 2016, 2, 1500344. (31) Tuinstra, F.; Koenig, J. L. Raman Spectrum of Graphite J. Chem. Phys. 1970, 53, 11261130. (32) Chang, S. H.; Chiang, Ch.-H.; Kao, F.-S.; Tien, Ch.-L.; Wu, Ch.-G. Unraveling the Enhanced Electrical Conductivity of PEDOT:PSS Thin Films for ITO-Free Organic Photovoltaics, IEEE Photonics Journal 2014, 6, 8400307.

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(33) Chou, T.-R.; Chen, S.-H.; Chiang, Y.-T.; Lin, Y.-T.; Chao, Ch.-Y. Highly conductive PEDOT:PSS films by post-treatment with dimethyl sulfoxide for ITO-free liquid crystal display J. Mater. Chem. C, 2015, 3, 3760-3766. (34) Mott, N. F.; Devis, E. A. Electron processes in non-crystalline materials, Clarendon Press, Oxford, 1979. (35) Ozaki, S.; Wada, Y.; Noda, K. DC Hall-effect measurement for inkjet-deposited films of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) by using microscale gap electrodes Synthetic Metals 2016, 215, 28-34. (36) Mudd, G. W.; Svatek, S. A.; Ren, T.; Patane, A.; Makarovsky, O.; Eaves, L.; Beton, P. H.; Kovalyuk, Z. D.; Lashkarev, G. V.; Kudrynskyi, Z. R.; Dmitriev, A. I. Tuning the Bandgap of Exfoliated InSe Nanosheets by Quantum Confinement Adv. Mater., 2013, 25, 5714-5718. (37) Mudd, G. W.; Svatek, S. A.; Hague, L.; Makarovsky, O.; Kudrynskyi, Z. R.; Mellor, C. J.; Beton, P. H.; Eaves, L.; Novoselov, K. S.; Kovalyuk, Z. D.; Vdovin, E. E.; Marsden, A. J.; Wilson, N. R.; Patane, A. High Broad-Band Photoresponsivity of Mechanically Formed InSe–Graphene van der Waals Heterostructures Adv. Mater., 2015, 27, 3760-3766. (38) Bandurin, D. A.; Tyurnina, A. V.; Yu, G. L.; Mishchenko, A.; Zolyomi, V.; Morozov, S. V.; Kumar, R. K.; Gorbachev, R. V.; Kudrynskyi, Z. R.; Pezzini, S.; Kovalyuk, Z. D.; Zeitler, U.; Novoselov, K. S.; Patane, A.; Eaves, L.; Grigorieva, I. V.; Falko, V. I.; Geim, A. K.; Cao, Y. High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe Nature Nanotechnol., 2017, 12, 223-227. (39) Ikari, T.; Shigetomi, S.; Hashimoto, K. Crystal Structure and Raman Spectra of InSe Phys. Stat. Sol. (b), 1982, 111, 477-481.

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(40) Chen, Z.; Gacem, K.; Boukhicha, M.; Biscaras, J.; Shukla, A. Anodic bonded 2D semiconductors: from synthesis to device fabrication Nanotechnology, 2013, 24, 415708. (41) Brus, V. V.; Gluba, M. A.; Mai, Ch.-K.; Fronk, S. L.; Rappich, J.; Nickel, N. H.; Bazan, G. C. Conjugated Polyelectrolyte/Graphene Heterobilayer Nanocomposites Exhibit Temperature Switchable Type of Conductivity Adv. Electon. Mater. 2017, 3, 1600515.

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