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Investigation into Ex-Situ and In-Situ Iodine Doped Plasma Polymerized Fluorene-type Thin Films
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 1955–1963
www.materialstoday.com/proceedings
ICSMD-2017
Investigation into Ex-Situ and In-Situ Iodine Doped Plasma Polymerized Fluorene-type Thin Films Dogan Mansuroglua,b,*, Ilker Umit Uzun-Kaymaka a
b
Deparment of Physics, Middle East Technical University, Ankara-06800, Turkey Deparment of Physics, Canakkale Onsekiz Mart University, Canakkale-17100, Turkey
Abstract Effect of doping is investigated by applying iodine dopant to plasma polymerized fluorene (C13H10)-type thin films utilizing two separate techniques: ex-situ and in-situ doping. In this study, the thin films are produced using a mixture of biphenyl and methane in a capacitively coupled glow plasma system. Produced thin films are characterized using a fourier transform infrared spectrometer, an ultraviolet-visible spectrometer, a scanning electron microscope, and a two-point probe technique of the current – voltage characteristics. The results show that the in-situ doping technique is more effective than the ex-situ doping technique based on the improvements observed.
© 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INTERNATIONAL CONGRESS ON SEMICONDUCTOR MATERIALS AND DEVICES. Keywords: Plasma polymerized film; fluorene; ex-situ doping; in-situ doping; iodine.
1. Introduction Conjugated organic materials such as polythiophene, polyaniline, polyfluorene and their derivatives have received appreciable attention in the applications of chemical sensors, photovoltaics, light emitting diodes, optical coatings or biomedicine due to their excellent physical and chemical properties [1–4]. Dopant materials are most commonly used to improve the electrical properties of these conjugated polymers by changing the electronic equilibrium in the molecular structures and by increasing the charge transfer complexes in the chains [5]. The most known dopant
* Corresponding author. Tel.: +90-312-210-4330; fax: +90-312-210-5099. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INTERNATIONAL CONGRESS ON SEMICONDUCTOR MATERIALS AND DEVICES.
1956
D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
Nomenclature I2 PPF RF UV-vis FTIR SEM I–V α
iodine plasma polymerized fluorene radio frequency ultraviolet-visible fourier transform infrared scanning electron microscope current – voltage absorption coefficient
materials can be listed as iodine (I2), chlorine (Cl), hydrogen chloride (HCl) or iron trichloride (FeCl3) [6–10]. These materials can be applied to polymers using two separate techniques: the ex-situ and the in-situ doping [7,8,11,12]. In the ex-situ doping technique the dopant materials are added after producing the products whereas the dopants are directly applied during the process in the in-situ doping technique. In our study, the ex-situ and the in-situ doping techniques are applied to plasma polymerized fluorene (PPF)-type thin films and the effects of both techniques are separately investigated. The results are compared with each other to find the better doping technique for PPF-type thin films. Fluorene, a π-conjugated aromatic hydrocarbon, is a promising material in the research areas such as organic light emitting diodes, photovoltaic applications, and biosensors [13–15] due to its blue light-emitting property, good charge mobility, easy processing purposes [16,17]. I2 is used as the dopant material, which is one of the most commonly used dopant materials for polymers. It changes the electro-negativity efficiently, moreover, it is easy to control in plasma chamber given that it is compatible with other compounds [7,11,12]. PPF-type thin films are produced using a capacitively coupled radio frequency (RF) plasma system utilizing at mixture of biphenyl (C12H10) and methane (CH4). Biphenyl material is used as the source of aromatic ring [18]. In our previous work, the synthesis of PPF-type thin films and their characterization are investigated in details for the purposes of a different experimental set up [19]. This work focuses on the investigation into the doping effects of I2 to the PPF-type thin films under the ex-situ and the in-situ doping techniques. Optical and electrical characteristics of thin films are analyzed using an ultraviolet-visible (UV-vis) spectrometer. The thin films are chemically characterized using a fourier transform infrared (FTIR) spectrometer. The surface morphologies are evaluated using a scanning electron microscope (SEM). The electrical conductivity is measured using the two-point probe method. 2. Experimental Details 2.1. Plasma chamber and materials The plasma chamber consists of two parallel cylindrical-steel electrodes where a 13.56 MHz RF power is applied between these electrodes, as shown in Fig. 1. Film depositions are produced on glass and silicon substrates. These substrates are cleaned with ethanol and acetone. A MKSTM multi-gas controller 647C is used to introduce methane into the chamber. Biphenyl, purchased in solid phase from Sigma-Aldrich Company (ReagentPlus®, 99%, melting at 69 °C), is placed into a heated reservoir at 130 °C. The pressure of the chamber is first dropped to 5x10-3 torr. When the biphenyl is vaporized sufficiently, the volatile biphenyl is mixed with the methane and the mixture is introduced to the vacuum chamber. The pressure is kept at 0.2 torr. The plasma is generated using a capacitively coupled RF source. Depositions are obtained at the RF input power values of 100 W and 200 W. The CH4 flow rate is kept at 5 sccm and the deposition time is fixed to 20 min.
D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
1957
Fig. 1. Capacitively coupled RF plasma system.
2.2. Descriptions of ex-situ and in-situ doping methods During the ex-situ doping technique, the crystalline iodine (purchased from Sigma Aldrich Company, ACS reagent, ≥99.8%) is placed at bottom of a desiccator. Since the iodine is volatile at atmospheric pressure, the dessicator is filled with the volatile iodine. The substrates are put into this desiccator after the deposition and they are doped for different time periods such as 24 h, 48 h, and 70 h. On the other case, during the in-situ doping technique, the iodine is placed into a separate reservoir near the biphenyl reservoir and heated up to 200 °C. The vaporized iodine is introduced into the chamber and thus it is directly mixed with the biphenyl and the methane plasma. The iodine radicals interact with the fragments of the polymer structures in the chamber and then the iodinated functional structures are generated. 2.3. Measurements Optical transmittance spectra of thin films are measured using a Perkin Elmer Lambda 45 UV-vis spectrometer with a range of 190 – 1100 nm and a spectral bandwidth of 0.5 nm. The chemical structures of the films are investigated using a Bruker IFS 66/s FTIR spectrometer with 4 cm-1 spectral resolution and its peak to peak noise <10-5 AU within a minute. A QUANTA 400F Field Emission scanning electron microscope with 1.2 nm resolution is used to analyze the surface morphologies of the thin films. The current – voltage (I – V) characteristics are evaluated using a Keithley 2401 power supply. 3. Results and Discussion 3.1. UV-vis spectrometer The optical performance of the undoped and I2 doped thin films are measured using a UV-vis spectrometer and the absorbance is analyzed in the wavelength range of 300 nm to 900 nm. The absorption coefficient (α) is defined as a function of the photon energy. As seen in Fig. 2, the results of the ex-situ doped films show that the absorption peak attributed ππ* excitation is presented around 364 nm after the doping. As the produced PPF-type thin films are exposed to I2 dopant materials using the ex-situ doping technique, the I2 is absorbed on the surfaces of polymer films and interacts with unsaturated compounds and the radicals. Interactions with the electrons from the orbitals increase the hole density of the films and this causes the formation of an additional level in the band structure as seen in the results. Additionally, the peaks appear more clearly with the increased deposition time and a small shift to higher wavelength is observed. Since the π-π* transition is related to the conjugation property of the film
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D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
structure, it can be suggested that the conjugation length increases with increased deposition time, especially for PPF-type thin films produced at 200 W.
Fig. 2. UV-vis spectra of the undoped and the ex-situ doped films produced at the RF input power of (a) 100 W; (b) 200 W.
Fig. 3 shows that the α calculated from the UV-vis spectra for the undoped and the in-situ doped PPF-type thin films. The absorbance peak of the π-π* transition is observed around 372 nm. In the in-situ doping technique the dopants directly interact with the bulk structures of the polymers. Iodine increases the electro-negativity of the polymer and this causes more interactions between the PPF molecules and the radicals of I2. In the spectra, the absorption peak shifts to higher wavelengths and a broader absorption band is observed with increasing RF input power. This shows that the conjugation property of the films increases with the RF power. The red shift can also be related to the high crosslinking degrees of the structures [11]. The conjugation length depends on the charge transfer complexes. The enhancement in the conjugation length confirms the incorporation of I2 and the presence of (PPF)+Icharge complexes in the film structures [11].
Fig. 3. UV-vis spectra of the undoped and the in-situ doped films.
D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
1959
The optical energy band gaps of thin films are calculated from the Tauc`s relation of the form [20] /
(1)
where , , and are the photon energy, the optical energy band gap, and a constant, respectively. The values are calculated from the extrapolation of vs plot taken for 0. The measured values for both doping techniques are shown in Fig. 4. The optical band gaps of the undoped films are observed at 3.51 eV and 3.48 eV at the RF input power of 100 W and 200 W, respectively. The values of the film after 24 h, 48 h, and 70 h I2 doping, at 100 W RF power decrease to 3.1 eV, 2.98 eV, and 2.96 eV; whereas, at 200 W RF power, these bands are observed at 3 eV, 2.9 eV, and 2.87 eV, respectively. During the ex-situ doping, almost no significant decrease is observed after passing the 48 h doping time limit. In other words, increasing the doping time does not supply a continuous decrease in the values. This can be due to the limited interactions and saturation effects between the absorbed I2 atoms and the surface of the polymer. The surface of the PPF-type thin film reaches an equilibrium in a certain doping time with respect to existing radicals and unsaturated compounds. However, a considerable decrease is observed in the in-situ doping technique where the calculated values are 2.69 eV at 100 W and 2.31 eV at 200 W. This shows that the (PPF)+ ions combine with the charged species of I2 atoms and effectively improve the electrical characteristics of the PPF-type thin films via providing the additional charge complexes [21]. Increasing the RF input power causes a lower band gap value, presumably by generating more excited fragmentations during the deposition and more iodinated compounds in the film structures. The UV-vis results exhibit that the electrical characteristics of the PPF-type thin films are more efficiently improved using the in-situ doping technique compared to the ex-situ doping.
Fig. 4. The
values of the ex-situ and the in-situ doped films.
3.2. FTIR spectrometer The infrared spectra of thin films are conducted for a wavenumber range of 3300 cm-1 – 500 cm-1 to evaluate the chemical properties before and after the doping. Since higher number of fragmentation occur during the plasma deposition, the PPF-type thin films show high crosslinking and branched structures which confirm with the broad band peaks in the spectra. The aliphatic and the asymmetric C-H stretching vibration bond of methyl groups are obtained at 2920 cm-1 and 2800 cm-1 [11,12,22]. The bands referring to the aromatic structures are observed at 1603 cm-1 as well as at 3100 cm-1. These bands are assigned to the aromatic C=C groups and the aromatic C-H stretching vibration, respectively [11,12]. The intensities of these aromatic bands increase in the doped films. In particular a sharp increase is obtained in the spectra observed for the in-situ doped film. Here, it can be suggested many conjugated segments exist in the chains of the films [12]. Additionally, the C-C stretching vibration is obtained at 2218 cm-1. The intensity of this band is also significantly increased in the spectra observed for the in-situ doped
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film. It can be said that the number densities of the C=C and the C-C bonds in the doped films are higher than those of the undoped. Since the number densities of the single and the double C bonds are directly related to the quality of the film [23,24], it can be suggested that the in-situ doped thin films have better film quality. Moreover, the band at 2490 cm-1 referring to the OH stretching vibration shows the presence of the oxygen groups in structures of the films. The reactions of the plasma macromolecular structures with the oxygen or the water vapor in the air can produce these oxygen derivatives [19,25]. The presence of the iodinated compounds is verified by the band observed at 1436 cm-1 assigned to –CH2-Igroups [7,22]. For the in-situ doped films, these groups exhibit stronger bands than that of the ex-situ doped films. Direct interaction of I2 with the polymer structure causes recombination and formation of iodinated compounds. The band peaks from 850 cm-1 to 656 cm-1 show that aromatic rings in the film structures react with the I2 atoms and generate the substitutions of the aromatic structures with the I2 bonds [12,26]. The presence of these bands can be accepted as a strong evidence of the formation of the I2 side-reactions suggested by Groenewoud et al [7]. According to the side-reactions, the I2 increases the electro-negativity of the PPF molecules and its radicals start to remove the hydrogen atoms from the benzene rings. The broken hydrogen atoms connect with I2 radicals and form HI molecules. These formed HI molecules then interact with other CH groups in the reactions. The PPF radicals presented in the reactions interact with other I2 radicals and generate the mono-substituted compounds. These peaks of I2 substitutions do not appear in the spectra of the ex-situ I2 doped film which should be due to the limited reactions on the surface. From the spectra, it is observed that the formations of the iodinated compounds supply a higher number density of aromatic compounds. Therefore, it can be said that applying the I2 directly to the PPF-type films and doping with the in-situ technique provides better chemical properties, higher the quality and the stability compared to those doped ex-situ. The observed FTIR results are in good agreement with the results of the electrical characteristics found in the UV-vis spectra, as shown in Fig. 5.
Fig. 5. FTIR spectra of the undoped, the ex-situ and the in-situ doped films.
3.3. SEM measurements The surface morphologies of the thin films are investigated using the SEM measurements. The undoped thin film has a smooth surface as shown in Fig. 6a. Fig. 6b shows that the thin films have no significant changes in the morphology after the ex-situ doping, only some weak globules are observed. The surface of the in-situ doped films deposited at 100 W has micro and nano structures as well as irregular aggregations of these structures. As the RF input power increases, the film presents a continuous surface morphology with more globular structures as shown in
D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
1961
Fig. 6d. It can be said that the incorporation of I2 with the PPF structures causes changes in the surface morphologies, especially after the in-situ doping technique, which is consistent with the FTIR results.
Fig. 6. SEM images of (a) undoped; (b) the ex-situ doped; (c), (d) the in-situ doped films. (All images are obtained at magnification of 24000 X and scale bar is 4 µm.)
3.4. I – V characterization The I –V characteristics of the thin films are measured using a two-point probe method. The surfaces of the films are coated by Al material to obtain probe-contact points with thickness about 100 nm. The points are configured as circles with 1 mm diameter and 1 mm separation. All I – V measurements are conducted between -30 V and 30 V and the electrical conductivity values are calculated from the Ohm`s relation using the slope of the I – V plot. The measured electrical conductivity of the undoped film is 2.5E-6 Scm-1. These measurements are repeated both in the dark and while the doped films are exposed to a broadband light source. Almost no significant change is observed in the conductivity for the ex-situ doped films as compared to that of the undoped film. On the other hand, the in-situ doped films show ohmic characteristics and the conductivities measured in the dark are 3.94E-5 Scm-1 and 1.76E-4 Scm-1 for the RF input power of 100 W and 200 W, respectively. This shows that the direct interaction of the electro-negative I2 atoms with the aromatic compounds improves the electrical transportation in the PPF structures due to the increase in the charge transfer complexes [7]. The maximum conductivity of the PPF thin film is obtained about two orders of magnitude higher than that of the undoped one. As the films are measured in the light, the conductivity measurements show a remarkable increase and the values are, respectively 8.42E-5 Scm-1 and 7.72E-4 Scm-1. Furthermore, as for the doped films placed under the light, the photons may decompose the I2 atoms and cause the formation of I2 radicals [7]. This sensitivity to the light makes the I2 doped PPF thin films much attractive for future applications such as photodetectors.
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D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
4. Conclusion The produced PPF thin films are doped using the I2 dopant material via both the ex-situ and the in-situ doping techniques. The improvements in the electrical and the chemical properties of the films are discussed in detail. The results of the UV-vis spectra show that the in-situ doped films have better electrical properties than the ex-situ doped ones. The calculated optical energy band gaps decrease as low as 2.31 eV using the in-situ doping technique. Chemically, the I2 causes the formations of the side-reactions as applied directly to the PPF films during the deposition. These side-reactions do not appear in the FTIR spectra observed for the ex-situ doped films. It can be suggested that the in-situ doped technique leads to an increase in the film quality at a higher degree than the ex-situ technique. The changes in the surface morphologies after the doping are evaluated from the SEM images. No significant change is observed in the morphology of the ex-situ doped film while micro/nano globular structures are observed in that of the in-situ doped films. Also, a continuous surface with very globular structures is obtained with increasing the RF input power. Moreover, the conductivity is calculated from the I – V measurements. The conductivity of the in-situ doped films is observed to be about two orders of magnitude higher than that of the undoped film. More importantly, these films react to the photons as they are measured in the light background and the conductivity increases significantly. The ex-situ doped films do not show any change in the conductivity. The results show that the improvements obtained using the in-situ doping technique are more efficient. Acknowledgements This research is supported by the Scientific Research Project Fund of Middle East Technical University BAP-0105-2017-006 and BAP-08-11-2017-040. References [1] Leclerc M. Polyfluorenes: Twenty years of progress. J Polym Sci Part A Polym Chem. 2001;39(17):2867–73. [2] Mcquade DT, Pullen AE, Swager TM. Conjugated polymer-based chemical sensors. Chem Rev. 2000;100:2537–74. [3] Perepichka BIF, Perepichka DF, Meng H, Wudl F. Light-emitting polythiophenes**. Adv Mater. 2005;17:2281–305. [4] Friend RH, Gymer RW, Holmes AB, Burroughes JH, Marks RN, Taliani C, et al. Electroluminescence in conjugated polymers. Nature. 1999;397:121–8. [5] Denes FS, Manolache S. Macromolecular plasma-chemistry: An emerging field of polymer science. Prog Polym Sci. 2004;29(8):815–85. [6] Silverstein MS, Visoly-Fisher I. Plasma polymerized thiophene : molecular structure and electrical properties. Polymer. 2002;43:11–20. [7] Groenewoud LMH, Engbers GHM, White R, Feijen J. On the iodine doping process of plasma polymerised thiophene layers. Synth Met. 2002;125(3):429–40. [8] Park C, Kim DY, Kim DH, Lee H, Shin BJ, Tae H-S. Humidity-independent conducting polyaniline films synthesized using advanced atmospheric pressure plasma polymerization with in-situ iodine doping. Appl Phys Lett. 2017;110:33502. [9] Mansuroglu D, Uzun-Kaymak IU. Enhancement of electrical conductivity of plasma polymerized fluorene-type thin film under iodine and chlorine dopants. Thin Solid Films. 2017;636:773–8. [10] Vásquez M, Cruz GJ, Olayo MG, Timoshina T, Morales J, Olayo R. Chlorine dopants in plasma synthesized heteroaromatic polymers. Polymer. 2006;47(23):7864–70. [11] Paosawatyanyong B, Kamphiranon P, Bannarakkul W, Srithana-anant Y, Bhanthumnavin W. Doping of polythiophene by microwave plasma deposition. Surf Coatings Technol. 2010;204(18–19):3053–8. [12] Olayo MG, Morales J, Cruz GJ, Olayo R, Ordoñez E, Barocio SR. On the influence of electron energy on iodine-doped polyaniline formation by plasma polymerization. J Polym Sci Part B Polym Phys. 2001;39(1):175–83. [13] Bernius M, Inbasekaran M, Woo E, Wu W, Wujkowski L. Light-emitting diodes based on fluorene polymers. Thin Solid Films. 2000;363:55–7. [14] Bundgaard E, Krebs F. Low band gap polymers for organic photovoltaics. Sol Energy Mater Sol Cells. 2007;91(11):954–85. [15] Huang F, Wang X, Wang D, Yang W, Cao Y. Synthesis and properties of a novel water-soluble anionic polyfluorenes for highly sensitive biosensors. Polymer. 2005;46(25):12010–5. [16] Inbasekaran M, Woo E, Wu W, Bernius M, Wujkowski L. Fluorene homopolymers and copolymers. Synth Met. 2000 Jun;111–112:397– 401. [17] Grimsdale AC, Müllen K. Polyphenylene-type emissive materials: poly (para-phenylene)s, polyfluorenes, and ladder polymers. Adv Polym Sci. 2006;199:1–82. [18] Friedrich J. Mechanisms of plasma polymerization - reviewed from a chemical point of view. Plasma Process Polym. 2011;8(9):783–802. [19] Mansuroglu D, Manolache S. Synthesis of fluorene-type thin film under biphenyl/methane plasma environment. Plasma Process Polym. 2015;12(10):1036–49. [20] Tauc J. Optical properties of amorphous semiconductors. In: Tauc J, editor. Amorphous and liquid semiconductors. London, New York: Plenum Press; 1974.
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[21] Omer AMM, Adhikari S, Adhikary S, Uchida H, Umeno M. Photovoltaic characteristics of postdeposition iodine-doped amorphous carbon films by microwave surface wave plasma chemical vapor deposition. Appl Phys Lett. 2005;87(16):1–3. [22] Wang J, Neoh KG, Kang ET. Comparative study of chemically synthesized and plasma polymerized pyrrole and thiophene thin films. Thin Solid Films. 2004;446(2):205–17. [23] Yasuda H. Plasma polymerization. Orlando: Academic Press; 1985. [24] Ashfold MNR, May PW, Rego CA, Everitt NM. Thin film diamond by chemical vapour deposition methods. Chem Soc Rev. 1994;23(1):21. [25] Biederman H. Plasma polymer films. London: Imperial College Press; 2004. [26] Morales J, Olayo MG, Cruz GJ, Olayo R. Plasma polymerization of random polyaniline-polypyrrole-iodine copolymers. J Appl Polym Sci. 2002;85(2):263–70.
ScienceDirect Materials Today: Proceedings 18 (2019) 1955–1963
www.materialstoday.com/proceedings
ICSMD-2017
Investigation into Ex-Situ and In-Situ Iodine Doped Plasma Polymerized Fluorene-type Thin Films Dogan Mansuroglua,b,*, Ilker Umit Uzun-Kaymaka a
b
Deparment of Physics, Middle East Technical University, Ankara-06800, Turkey Deparment of Physics, Canakkale Onsekiz Mart University, Canakkale-17100, Turkey
Abstract Effect of doping is investigated by applying iodine dopant to plasma polymerized fluorene (C13H10)-type thin films utilizing two separate techniques: ex-situ and in-situ doping. In this study, the thin films are produced using a mixture of biphenyl and methane in a capacitively coupled glow plasma system. Produced thin films are characterized using a fourier transform infrared spectrometer, an ultraviolet-visible spectrometer, a scanning electron microscope, and a two-point probe technique of the current – voltage characteristics. The results show that the in-situ doping technique is more effective than the ex-situ doping technique based on the improvements observed.
© 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INTERNATIONAL CONGRESS ON SEMICONDUCTOR MATERIALS AND DEVICES. Keywords: Plasma polymerized film; fluorene; ex-situ doping; in-situ doping; iodine.
1. Introduction Conjugated organic materials such as polythiophene, polyaniline, polyfluorene and their derivatives have received appreciable attention in the applications of chemical sensors, photovoltaics, light emitting diodes, optical coatings or biomedicine due to their excellent physical and chemical properties [1–4]. Dopant materials are most commonly used to improve the electrical properties of these conjugated polymers by changing the electronic equilibrium in the molecular structures and by increasing the charge transfer complexes in the chains [5]. The most known dopant
* Corresponding author. Tel.: +90-312-210-4330; fax: +90-312-210-5099. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INTERNATIONAL CONGRESS ON SEMICONDUCTOR MATERIALS AND DEVICES.
1956
D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
Nomenclature I2 PPF RF UV-vis FTIR SEM I–V α
iodine plasma polymerized fluorene radio frequency ultraviolet-visible fourier transform infrared scanning electron microscope current – voltage absorption coefficient
materials can be listed as iodine (I2), chlorine (Cl), hydrogen chloride (HCl) or iron trichloride (FeCl3) [6–10]. These materials can be applied to polymers using two separate techniques: the ex-situ and the in-situ doping [7,8,11,12]. In the ex-situ doping technique the dopant materials are added after producing the products whereas the dopants are directly applied during the process in the in-situ doping technique. In our study, the ex-situ and the in-situ doping techniques are applied to plasma polymerized fluorene (PPF)-type thin films and the effects of both techniques are separately investigated. The results are compared with each other to find the better doping technique for PPF-type thin films. Fluorene, a π-conjugated aromatic hydrocarbon, is a promising material in the research areas such as organic light emitting diodes, photovoltaic applications, and biosensors [13–15] due to its blue light-emitting property, good charge mobility, easy processing purposes [16,17]. I2 is used as the dopant material, which is one of the most commonly used dopant materials for polymers. It changes the electro-negativity efficiently, moreover, it is easy to control in plasma chamber given that it is compatible with other compounds [7,11,12]. PPF-type thin films are produced using a capacitively coupled radio frequency (RF) plasma system utilizing at mixture of biphenyl (C12H10) and methane (CH4). Biphenyl material is used as the source of aromatic ring [18]. In our previous work, the synthesis of PPF-type thin films and their characterization are investigated in details for the purposes of a different experimental set up [19]. This work focuses on the investigation into the doping effects of I2 to the PPF-type thin films under the ex-situ and the in-situ doping techniques. Optical and electrical characteristics of thin films are analyzed using an ultraviolet-visible (UV-vis) spectrometer. The thin films are chemically characterized using a fourier transform infrared (FTIR) spectrometer. The surface morphologies are evaluated using a scanning electron microscope (SEM). The electrical conductivity is measured using the two-point probe method. 2. Experimental Details 2.1. Plasma chamber and materials The plasma chamber consists of two parallel cylindrical-steel electrodes where a 13.56 MHz RF power is applied between these electrodes, as shown in Fig. 1. Film depositions are produced on glass and silicon substrates. These substrates are cleaned with ethanol and acetone. A MKSTM multi-gas controller 647C is used to introduce methane into the chamber. Biphenyl, purchased in solid phase from Sigma-Aldrich Company (ReagentPlus®, 99%, melting at 69 °C), is placed into a heated reservoir at 130 °C. The pressure of the chamber is first dropped to 5x10-3 torr. When the biphenyl is vaporized sufficiently, the volatile biphenyl is mixed with the methane and the mixture is introduced to the vacuum chamber. The pressure is kept at 0.2 torr. The plasma is generated using a capacitively coupled RF source. Depositions are obtained at the RF input power values of 100 W and 200 W. The CH4 flow rate is kept at 5 sccm and the deposition time is fixed to 20 min.
D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
1957
Fig. 1. Capacitively coupled RF plasma system.
2.2. Descriptions of ex-situ and in-situ doping methods During the ex-situ doping technique, the crystalline iodine (purchased from Sigma Aldrich Company, ACS reagent, ≥99.8%) is placed at bottom of a desiccator. Since the iodine is volatile at atmospheric pressure, the dessicator is filled with the volatile iodine. The substrates are put into this desiccator after the deposition and they are doped for different time periods such as 24 h, 48 h, and 70 h. On the other case, during the in-situ doping technique, the iodine is placed into a separate reservoir near the biphenyl reservoir and heated up to 200 °C. The vaporized iodine is introduced into the chamber and thus it is directly mixed with the biphenyl and the methane plasma. The iodine radicals interact with the fragments of the polymer structures in the chamber and then the iodinated functional structures are generated. 2.3. Measurements Optical transmittance spectra of thin films are measured using a Perkin Elmer Lambda 45 UV-vis spectrometer with a range of 190 – 1100 nm and a spectral bandwidth of 0.5 nm. The chemical structures of the films are investigated using a Bruker IFS 66/s FTIR spectrometer with 4 cm-1 spectral resolution and its peak to peak noise <10-5 AU within a minute. A QUANTA 400F Field Emission scanning electron microscope with 1.2 nm resolution is used to analyze the surface morphologies of the thin films. The current – voltage (I – V) characteristics are evaluated using a Keithley 2401 power supply. 3. Results and Discussion 3.1. UV-vis spectrometer The optical performance of the undoped and I2 doped thin films are measured using a UV-vis spectrometer and the absorbance is analyzed in the wavelength range of 300 nm to 900 nm. The absorption coefficient (α) is defined as a function of the photon energy. As seen in Fig. 2, the results of the ex-situ doped films show that the absorption peak attributed ππ* excitation is presented around 364 nm after the doping. As the produced PPF-type thin films are exposed to I2 dopant materials using the ex-situ doping technique, the I2 is absorbed on the surfaces of polymer films and interacts with unsaturated compounds and the radicals. Interactions with the electrons from the orbitals increase the hole density of the films and this causes the formation of an additional level in the band structure as seen in the results. Additionally, the peaks appear more clearly with the increased deposition time and a small shift to higher wavelength is observed. Since the π-π* transition is related to the conjugation property of the film
1958
D. Mansuroglu and İ. U.Uzun-Kaymak / Materials Today: Proceedings 18 (2019) 1955–1963
structure, it can be suggested that the conjugation length increases with increased deposition time, especially for PPF-type thin films produced at 200 W.
Fig. 2. UV-vis spectra of the undoped and the ex-situ doped films produced at the RF input power of (a) 100 W; (b) 200 W.
Fig. 3 shows that the α calculated from the UV-vis spectra for the undoped and the in-situ doped PPF-type thin films. The absorbance peak of the π-π* transition is observed around 372 nm. In the in-situ doping technique the dopants directly interact with the bulk structures of the polymers. Iodine increases the electro-negativity of the polymer and this causes more interactions between the PPF molecules and the radicals of I2. In the spectra, the absorption peak shifts to higher wavelengths and a broader absorption band is observed with increasing RF input power. This shows that the conjugation property of the films increases with the RF power. The red shift can also be related to the high crosslinking degrees of the structures [11]. The conjugation length depends on the charge transfer complexes. The enhancement in the conjugation length confirms the incorporation of I2 and the presence of (PPF)+Icharge complexes in the film structures [11].
Fig. 3. UV-vis spectra of the undoped and the in-situ doped films.
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The optical energy band gaps of thin films are calculated from the Tauc`s relation of the form [20] /
(1)
where , , and are the photon energy, the optical energy band gap, and a constant, respectively. The values are calculated from the extrapolation of vs plot taken for 0. The measured values for both doping techniques are shown in Fig. 4. The optical band gaps of the undoped films are observed at 3.51 eV and 3.48 eV at the RF input power of 100 W and 200 W, respectively. The values of the film after 24 h, 48 h, and 70 h I2 doping, at 100 W RF power decrease to 3.1 eV, 2.98 eV, and 2.96 eV; whereas, at 200 W RF power, these bands are observed at 3 eV, 2.9 eV, and 2.87 eV, respectively. During the ex-situ doping, almost no significant decrease is observed after passing the 48 h doping time limit. In other words, increasing the doping time does not supply a continuous decrease in the values. This can be due to the limited interactions and saturation effects between the absorbed I2 atoms and the surface of the polymer. The surface of the PPF-type thin film reaches an equilibrium in a certain doping time with respect to existing radicals and unsaturated compounds. However, a considerable decrease is observed in the in-situ doping technique where the calculated values are 2.69 eV at 100 W and 2.31 eV at 200 W. This shows that the (PPF)+ ions combine with the charged species of I2 atoms and effectively improve the electrical characteristics of the PPF-type thin films via providing the additional charge complexes [21]. Increasing the RF input power causes a lower band gap value, presumably by generating more excited fragmentations during the deposition and more iodinated compounds in the film structures. The UV-vis results exhibit that the electrical characteristics of the PPF-type thin films are more efficiently improved using the in-situ doping technique compared to the ex-situ doping.
Fig. 4. The
values of the ex-situ and the in-situ doped films.
3.2. FTIR spectrometer The infrared spectra of thin films are conducted for a wavenumber range of 3300 cm-1 – 500 cm-1 to evaluate the chemical properties before and after the doping. Since higher number of fragmentation occur during the plasma deposition, the PPF-type thin films show high crosslinking and branched structures which confirm with the broad band peaks in the spectra. The aliphatic and the asymmetric C-H stretching vibration bond of methyl groups are obtained at 2920 cm-1 and 2800 cm-1 [11,12,22]. The bands referring to the aromatic structures are observed at 1603 cm-1 as well as at 3100 cm-1. These bands are assigned to the aromatic C=C groups and the aromatic C-H stretching vibration, respectively [11,12]. The intensities of these aromatic bands increase in the doped films. In particular a sharp increase is obtained in the spectra observed for the in-situ doped film. Here, it can be suggested many conjugated segments exist in the chains of the films [12]. Additionally, the C-C stretching vibration is obtained at 2218 cm-1. The intensity of this band is also significantly increased in the spectra observed for the in-situ doped
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film. It can be said that the number densities of the C=C and the C-C bonds in the doped films are higher than those of the undoped. Since the number densities of the single and the double C bonds are directly related to the quality of the film [23,24], it can be suggested that the in-situ doped thin films have better film quality. Moreover, the band at 2490 cm-1 referring to the OH stretching vibration shows the presence of the oxygen groups in structures of the films. The reactions of the plasma macromolecular structures with the oxygen or the water vapor in the air can produce these oxygen derivatives [19,25]. The presence of the iodinated compounds is verified by the band observed at 1436 cm-1 assigned to –CH2-Igroups [7,22]. For the in-situ doped films, these groups exhibit stronger bands than that of the ex-situ doped films. Direct interaction of I2 with the polymer structure causes recombination and formation of iodinated compounds. The band peaks from 850 cm-1 to 656 cm-1 show that aromatic rings in the film structures react with the I2 atoms and generate the substitutions of the aromatic structures with the I2 bonds [12,26]. The presence of these bands can be accepted as a strong evidence of the formation of the I2 side-reactions suggested by Groenewoud et al [7]. According to the side-reactions, the I2 increases the electro-negativity of the PPF molecules and its radicals start to remove the hydrogen atoms from the benzene rings. The broken hydrogen atoms connect with I2 radicals and form HI molecules. These formed HI molecules then interact with other CH groups in the reactions. The PPF radicals presented in the reactions interact with other I2 radicals and generate the mono-substituted compounds. These peaks of I2 substitutions do not appear in the spectra of the ex-situ I2 doped film which should be due to the limited reactions on the surface. From the spectra, it is observed that the formations of the iodinated compounds supply a higher number density of aromatic compounds. Therefore, it can be said that applying the I2 directly to the PPF-type films and doping with the in-situ technique provides better chemical properties, higher the quality and the stability compared to those doped ex-situ. The observed FTIR results are in good agreement with the results of the electrical characteristics found in the UV-vis spectra, as shown in Fig. 5.
Fig. 5. FTIR spectra of the undoped, the ex-situ and the in-situ doped films.
3.3. SEM measurements The surface morphologies of the thin films are investigated using the SEM measurements. The undoped thin film has a smooth surface as shown in Fig. 6a. Fig. 6b shows that the thin films have no significant changes in the morphology after the ex-situ doping, only some weak globules are observed. The surface of the in-situ doped films deposited at 100 W has micro and nano structures as well as irregular aggregations of these structures. As the RF input power increases, the film presents a continuous surface morphology with more globular structures as shown in
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Fig. 6d. It can be said that the incorporation of I2 with the PPF structures causes changes in the surface morphologies, especially after the in-situ doping technique, which is consistent with the FTIR results.
Fig. 6. SEM images of (a) undoped; (b) the ex-situ doped; (c), (d) the in-situ doped films. (All images are obtained at magnification of 24000 X and scale bar is 4 µm.)
3.4. I – V characterization The I –V characteristics of the thin films are measured using a two-point probe method. The surfaces of the films are coated by Al material to obtain probe-contact points with thickness about 100 nm. The points are configured as circles with 1 mm diameter and 1 mm separation. All I – V measurements are conducted between -30 V and 30 V and the electrical conductivity values are calculated from the Ohm`s relation using the slope of the I – V plot. The measured electrical conductivity of the undoped film is 2.5E-6 Scm-1. These measurements are repeated both in the dark and while the doped films are exposed to a broadband light source. Almost no significant change is observed in the conductivity for the ex-situ doped films as compared to that of the undoped film. On the other hand, the in-situ doped films show ohmic characteristics and the conductivities measured in the dark are 3.94E-5 Scm-1 and 1.76E-4 Scm-1 for the RF input power of 100 W and 200 W, respectively. This shows that the direct interaction of the electro-negative I2 atoms with the aromatic compounds improves the electrical transportation in the PPF structures due to the increase in the charge transfer complexes [7]. The maximum conductivity of the PPF thin film is obtained about two orders of magnitude higher than that of the undoped one. As the films are measured in the light, the conductivity measurements show a remarkable increase and the values are, respectively 8.42E-5 Scm-1 and 7.72E-4 Scm-1. Furthermore, as for the doped films placed under the light, the photons may decompose the I2 atoms and cause the formation of I2 radicals [7]. This sensitivity to the light makes the I2 doped PPF thin films much attractive for future applications such as photodetectors.
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4. Conclusion The produced PPF thin films are doped using the I2 dopant material via both the ex-situ and the in-situ doping techniques. The improvements in the electrical and the chemical properties of the films are discussed in detail. The results of the UV-vis spectra show that the in-situ doped films have better electrical properties than the ex-situ doped ones. The calculated optical energy band gaps decrease as low as 2.31 eV using the in-situ doping technique. Chemically, the I2 causes the formations of the side-reactions as applied directly to the PPF films during the deposition. These side-reactions do not appear in the FTIR spectra observed for the ex-situ doped films. It can be suggested that the in-situ doped technique leads to an increase in the film quality at a higher degree than the ex-situ technique. The changes in the surface morphologies after the doping are evaluated from the SEM images. No significant change is observed in the morphology of the ex-situ doped film while micro/nano globular structures are observed in that of the in-situ doped films. Also, a continuous surface with very globular structures is obtained with increasing the RF input power. Moreover, the conductivity is calculated from the I – V measurements. The conductivity of the in-situ doped films is observed to be about two orders of magnitude higher than that of the undoped film. More importantly, these films react to the photons as they are measured in the light background and the conductivity increases significantly. The ex-situ doped films do not show any change in the conductivity. The results show that the improvements obtained using the in-situ doping technique are more efficient. Acknowledgements This research is supported by the Scientific Research Project Fund of Middle East Technical University BAP-0105-2017-006 and BAP-08-11-2017-040. References [1] Leclerc M. Polyfluorenes: Twenty years of progress. J Polym Sci Part A Polym Chem. 2001;39(17):2867–73. [2] Mcquade DT, Pullen AE, Swager TM. Conjugated polymer-based chemical sensors. Chem Rev. 2000;100:2537–74. [3] Perepichka BIF, Perepichka DF, Meng H, Wudl F. Light-emitting polythiophenes**. Adv Mater. 2005;17:2281–305. [4] Friend RH, Gymer RW, Holmes AB, Burroughes JH, Marks RN, Taliani C, et al. Electroluminescence in conjugated polymers. Nature. 1999;397:121–8. [5] Denes FS, Manolache S. Macromolecular plasma-chemistry: An emerging field of polymer science. Prog Polym Sci. 2004;29(8):815–85. [6] Silverstein MS, Visoly-Fisher I. Plasma polymerized thiophene : molecular structure and electrical properties. Polymer. 2002;43:11–20. [7] Groenewoud LMH, Engbers GHM, White R, Feijen J. On the iodine doping process of plasma polymerised thiophene layers. Synth Met. 2002;125(3):429–40. [8] Park C, Kim DY, Kim DH, Lee H, Shin BJ, Tae H-S. Humidity-independent conducting polyaniline films synthesized using advanced atmospheric pressure plasma polymerization with in-situ iodine doping. Appl Phys Lett. 2017;110:33502. [9] Mansuroglu D, Uzun-Kaymak IU. Enhancement of electrical conductivity of plasma polymerized fluorene-type thin film under iodine and chlorine dopants. Thin Solid Films. 2017;636:773–8. [10] Vásquez M, Cruz GJ, Olayo MG, Timoshina T, Morales J, Olayo R. Chlorine dopants in plasma synthesized heteroaromatic polymers. Polymer. 2006;47(23):7864–70. [11] Paosawatyanyong B, Kamphiranon P, Bannarakkul W, Srithana-anant Y, Bhanthumnavin W. Doping of polythiophene by microwave plasma deposition. Surf Coatings Technol. 2010;204(18–19):3053–8. [12] Olayo MG, Morales J, Cruz GJ, Olayo R, Ordoñez E, Barocio SR. On the influence of electron energy on iodine-doped polyaniline formation by plasma polymerization. J Polym Sci Part B Polym Phys. 2001;39(1):175–83. [13] Bernius M, Inbasekaran M, Woo E, Wu W, Wujkowski L. Light-emitting diodes based on fluorene polymers. Thin Solid Films. 2000;363:55–7. [14] Bundgaard E, Krebs F. Low band gap polymers for organic photovoltaics. Sol Energy Mater Sol Cells. 2007;91(11):954–85. [15] Huang F, Wang X, Wang D, Yang W, Cao Y. Synthesis and properties of a novel water-soluble anionic polyfluorenes for highly sensitive biosensors. Polymer. 2005;46(25):12010–5. [16] Inbasekaran M, Woo E, Wu W, Bernius M, Wujkowski L. Fluorene homopolymers and copolymers. Synth Met. 2000 Jun;111–112:397– 401. [17] Grimsdale AC, Müllen K. Polyphenylene-type emissive materials: poly (para-phenylene)s, polyfluorenes, and ladder polymers. Adv Polym Sci. 2006;199:1–82. [18] Friedrich J. Mechanisms of plasma polymerization - reviewed from a chemical point of view. Plasma Process Polym. 2011;8(9):783–802. [19] Mansuroglu D, Manolache S. Synthesis of fluorene-type thin film under biphenyl/methane plasma environment. Plasma Process Polym. 2015;12(10):1036–49. [20] Tauc J. Optical properties of amorphous semiconductors. In: Tauc J, editor. Amorphous and liquid semiconductors. London, New York: Plenum Press; 1974.
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