porphyrin at room temperature

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Ferroeletric like characteristics in redox active polymer of 5,10,15,20 tetra(4-hydroxyphenyl)-porphyrin at room temperature S. P. Koiry, M. E. Celestin, R. Ratnadurai, P. Veerender, C. Majumder et al. Citation: Appl. Phys. Lett. 103, 033302 (2013); doi: 10.1063/1.4813736 View online: http://dx.doi.org/10.1063/1.4813736 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i3 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 103, 033302 (2013)

Ferroeletric like characteristics in redox active polymer of 5,10,15,20 tetra(4-hydroxyphenyl)-porphyrin at room temperature S. P. Koiry,1 M. E. Celestin,2 R. Ratnadurai,3 P. Veerender,1 C. Majumder,4 S. Krishnan,5 E. Stefanakos,3 Y. Goswami,2 D. K. Aswal,1,a) and Shekhar Bhansali5,a) 1

Technical Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India Chemical and Biomedical Engineering, Clean Energy Research Center, University of South Florida, Tampa, Florida 33613, USA 3 Electrical Engineering, University of South Florida, Tampa, Florida 33620, USA 4 Chemistry Division, Bhabha Atomic Research Center, Mumbai 400085, India 5 Electrical and Computer Engineering, Florida International University, Miami, Florida 33174, USA 2

(Received 18 March 2013; accepted 13 June 2013; published online 16 July 2013) We report ferroelectric behaviors in electrochemically polymerized 5,10,15,20 tetra(4-hydroxyphenyl)porphyrin. The ferroelectric behaviors are due to conformational changes that occur during the reduction and oxidation of the polymer under electric field. The conformational changes were studied by in situ Raman spectroscopy and frequency response analysis. The present findings will open up an alternative route for organic ferroelectrics which is presently in urgent need of approaches and C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4813736] materials. V

At present, ferroelectric-based random access memory devices (FeRAM) have emerged as advanced alternative non-volatile memory devices because of their low power consumption and high operational speed.1,2 Such use of ferroelectric materials is due to its hysterical nature of electrical characteristics, such as polarization vs voltage, capacitance vs voltage, and current vs voltage characteristics.3 However, to use ferroelectric materials in futuristic portable and stretchable devices, the materials should be light weight, flexible, and non-toxic.4 Such properties can be obtained from organicbased ferroelectric materials because organic materials offer easy processability, flexibility, and low weight.5 But limited availability of organic ferroelectric materials impedes the progress of research for ferroelectric-based futuristic devices.4 To date, there are very few organic ferroelectric materials, such as poly(vinylidenedifluoride), vinylidene fluoride oligomer, and thiourea.4,6 The scarcity of such organic materials is due to the difficulties in replicating the structures and properties of inorganic ferroelectric crystals in single organic molecular crystals.4,7 Consequently, alternative routes for synthesizing organic ferroelectric materials have been proposed, sought after, and demonstrated using multi-component systems.4,7 Among those multi-component systems, electron donor (D) and acceptor (A) molecular configuration have been proposed and demonstrated in tetrathiafulvalene (TTF) complexes formed between p-bromanil (tetrabromo-p-benzoquinone; BA) and p-chloranil (tetrachloro-p-benzoquinone).4,8 Ferroelectric properties in such systems were observed due to the neutral–ionic phase transition through intermolecular collective electron transfer.9 A chemical approach for designing organic ferroelectrics has also been considered using an acid–base combination through inter molecular hydrogen bonds.10,11 For example, ferroelectric behavior has been observed in the compounds of 5,50 -dimethyl-2,2-bipyridinebase and a)

Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected]

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iodanilic acid (1:1) formed by intermolecular hydrogen bonding between the O–H groups of one unit and the N–Hþ groups of another unit forming a polar chain.12 Under applied bias, all the protons are transferred simultaneously causing the reversal of the chain polarity without losing its chemical identity-which is a ferroelectric phenomenon.12 Such proton transfers also lead to change in bond length and bond angle of the molecules of the supramolecular structure.4 Although the multi components based approaches open up the path for synthesis of new organic ferroelectric materials, these materials are either insoluble in common organic solvents or non volatile which are the hindrance in the material processing, such as spin coating or thermal vapor deposition.4 Therefore, it is essential for a new approach in designing and synthesis of ferroelectric materials which can be deposited at ease. However, from the above-described multi-component systems, we realized that ferroelectric properties in such systems are based either on reversible electron transfer or protons transfer under applied bias, which is analogous to oxidation or reduction of the systems. Therefore, we anticipated that redox active polymers can be used for ferroelectricity and the advantage of such polymers is the possibilities of their synthesis by electrochemical method on the desired device area. In this article, we show ferroelectric behavior in redox active polymers of 5,10,15,20 tetra(4-hydroxyphenyl)-porphyrin (THPP) deposited on interdigitated gold electrodes at room temperature. For the present study, the polymerization of THPP (poly THPP) was first standardized on ITO and gold electrodes using a three-electrode electrochemical system. ITO or gold was used as the working electrode (WE), Pt as the counter electrode (CE), and Ag/AgCl as the reference electrode (RE). For polymerization, 1 mM 5,10,15,20 THPP (Aldrich) solution was prepared in methanol solution and was mixed with methanol solution of 0.1 M lithium perchlorite (LiClO4) (Aldrich) in 1:1(V/V) ratio. The polymerization was carried out by cylic voltammetry (CV) method. The CV scans were

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run from 0 V to 1 V at scan rate 0.05 Vs1 and potential step 0.05 V using Autolab system (PG stat 30). A brown coloration of the WE electrode was observed and the coloration intensified with the increase in number of scans. The polymer so formed was washed with methanol and was found to be insoluble in acetone, isopropanol, methanol, and water. The polymer was characterized by Raman spectroscopy measurements using Horiba JobinYvon HR800 at wavelength 632 nm and UV-vis spectroscopic studies (Jasco V 530). The surface morphology of the polymer was studied by scanning electron microscope (Hitachi SU70) and atomic force microscope (Bruker dimension 3000). Detailed characterization of polyTHPP has been discussed in Supplementary material, Sec. S1.20 After confirming polymerization, THPP was polymerized on IDE following same method of polymerization as discussed for gold. In this case, the only difference was that Interdigitated electrode (IDE) was used as the working electrode instead of gold electrode. Fabrication steps for IDE are explained in Supplementary material, Sec. S2. Optical image of the IDE was taken by Olympus BX4 attached with Raman spectrophotometer. DC measurements of the polymer on IDE were carried out using a Cascade Microtech probe station (source meter, Keithley 2400). For AC measurement, tungsten probes of 5 lm diameter were used to probe the samples. Sawyer–Tower circuit was used to measure charge-voltage hysteresis loop using a function generator and an oscilloscope (Techtronix TDS 3052). The change in capacitance due to oxidation and reduction of the polymer was carried out by frequency response analysis (FRA) using FRA module of Autolab system and data were analyzed using Z SIMPWIN 3.22 software. FRA was recorded using 10 mV signal and frequency in the range 1 MHz to 10 Hz at different DC voltages. Furthermore, to study in situ Raman spectra during the voltage cycle, tungsten microprobes were used for taking electrical contacts and the voltage was applied using the source meter. The dipole moment of the THPP monomer was calculated using MOPAC program (for details of theoretical calculation, see Supplementary material, Sec. S1). Figure 1 shows the CV response during THPP deposition on the IDE, and the inset illustrates the deposition of THPP before and after polymerization, confirming the contact between the electrodes. We observed that the evolution of CV during polymerization on IDE is similar to that observed for gold electrode (Fig. S1(a)).The polyTHPP on IDE was characterized by Raman spectroscopy measurements which showed characteristic bands of polymerized porphyrin (this will be discussed in detail later). The charge (Q) vs voltage (V) measurements of the polymer showed hysteresis loop like a ferroelectric capacitor at frequencies of 3 Hz, 5 Hz, 10 Hz, and 20 Hz and 5 V peak to peak voltage (Fig. 2(a) with the remnant charge of 0.065 mC.11,13,14 Since the hysteresis in (Q-V) plot alone cannot confirm ferroelectric properties, it is essential to investigate other properties, such as current vs. voltage (I-V) and capacitance–voltage (C-V) measurements related to ferroelectric materials.14 Figure 2(b) shows the existence of hysteresis in I-V characteristics of polyTHPP on IDE, and the inset in Fig. 2(b) (left inset) shows the I-V measurement

Appl. Phys. Lett. 103, 033302 (2013)

FIG. 1. CV of scan1, scan3, and scan5, respectively, measured during electrochemical polymerization of THPP on IDE as WE electrodes where CV was run from 0 V to þ1.0 V up to 5 scans and at a potential scan rate 0.05 V s1. Insets (1) and (2) show the optical images of IDE before and after electrochemical polymerization on it.

setup. These I-V characteristics show two current peaks at 1.1 V and 1 V, as well as the current at 0 V between forward and reverse bias cycle. These characteristics have a resemblance to the I-V characteristics of a ferroelectric material, such as BaTiO3/SrTiO3.14 We observed that the peak to valley ratio of the current varied from 5:1 to 8:1, and the peak position of current varied from 1 V to 1.5 V. These variations depend on the bridging between electrodes on which electron transport occurs. Furthermore, with an increase in the number of voltage cycles, the current slowly increased and even after 100 cycles, the I-V was not degraded (Fig. 2(b)). We also observed that peaks in I-V shifted towards low voltage with the number bias cycles and stabilized at around 1 V. Prior to the deposition of poly THPP, the electrical stability of each IDE was carried out by cycling the bias for 50 times in the bias range of 5 V to 5 V (left inset of Fig. 2(b)). The C-V characteristics were studied independently by FRA at different constant DC voltages and in the frequency range from 1 MHz to 1 Hz. Figure 3(a) shows the plot of Z00 vs applied bias from 0 V to 4.5 V and 0 V to 4.5 V at 10 Hz. This plot is nothing but capacitance (C) vs voltage (V) plot as Z00 ¼ 1/xC, where x is the frequency (Hz) and C is capacitance (F). Furthermore, the plot shows the clear two-Z00 peaks at 2 V and 2 V. Such capacitive peaks are characteristics of ferroelectric materials. The dielectric loss (tan d ¼ Z0 /Z00 ) of the film was varied from 0.4 to 16 in the bias range 0 V to 4.5 V. This variation can be understood from the change in impedance of the film with bias (impedance change will be discussed later). The electron transport in polyTHPP on IDE can be explained from its energy level diagram (see Supplementary material, Fig. S5). Briefly, at a negative bias (i.e., V < 1 V), the Fermi level of Au aligns with the LUMO lying at 3.9 eV causing the increase in current (for details, see Supplementary material, Sec. S1). Above 1 V, we believe that polymer undergoes reduction and this would cause changes in energy levels of polyTHPP. Thus, these changes in energy levels cause misalignment of Fermi levels

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FIG. 2. (a) Charge-voltage characteristics showing hysteric behavior which were measured using AC signal of frequencies of 3 Hz, 5 Hz, 10 Hz, 20 Hz, and 5 V peak to peak voltage. (b) Hysteric current voltage characteristics (I-V) of 10th, 50th, and 100th cycle, which have two current peaks at 1.0 V and 1.1 V. The left inset shows schematic device measurement setup, whereas the right inset shows I-V characteristics of IDE before deposition and was tested for 50 cycles. (c) The plot of Z00 vs applied bias from 0 V to 4.5 V and 0 V to 4.5 V at 10 Hz. The Z00 peaks are marked at 2 V and 2 V and arrows show the direction of voltage scan.

of Au. Consequently, a drop in current will occur, and this manifests itself as a peak in the I-V. Similarly, the positive current peak in the I-V can be explained from the alignment

FIG. 3. (a) Nyquist plot of imaginary impedance (Z00 ) vs real impedance (Z0 ) at 0 V, 2 V, and 2.5 V. (b) Raman spectra measured at 0 V and 2 V during the voltage cycles. Porphyrin skeleton bands are shown by arrows.

of the Fermi level of Au with the HOMO. Subsequently, oxidation of the polymer will occur during the positive cycle. Typically, negative differential resistance (current peak) of polymer or molecules in I-V characteristics has been explained by their redox properties.15 Similarly, the current peaks exhibited by the poly THPP films could also be correlated to the oxidation and reduction peaks (Supplementary material, Sec. S1). However, if the oxidation or reduction takes place under bias, then one would expect a change in conformation of polymer because addition or removal of electrons cause change in bond length or strength of any molecular system. Furthermore, this change in conformation would affect the impedance of the polymer. Therefore, the change in conformation can be studied by the impedance analysis and the Raman spectroscopy studies. Figure 3(a) shows the Nyquist plot of imaginary impedance (Z00 ) vs real impedance (Z0 ) at 0 V, 2 V, and 2.5 V. It is clearly seen that at 0 V, the film has capacitive behavior (semi circle) along with Warburg impedance (W). The Warburg impedance is related to the diffusion of charge in the film. Such impedance characteristics are observed for conducting polymers.16 Furthermore, the analysis of FRA data reveals that in the voltage range 2 V to 3 V, W changes to new circuit element; constant phase element(Q) in parallel with charge transfer resistance (Rct) (Supplementary material, Sec. S3). The development of the QR element indicates the oxidation and reduction of the film. Above 3 V, the impedance circuit shrinks to capacitance and resistance (RC) (Supplementary material, Sec. S3). Table I reveals that capacitance (Cg) is independent of the applied voltage—which confirms that Cg is geometrical capacitance. The interfacial resistance of the film decreases by six times at 2 V (427 to 70 kX), but the increase in resistance was observed at 2.5 V (142 kX). This explains the decrease in current—which

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TABLE I. Parameter values of fitting impedance data for polyTHPP on IDE to equivalent circuits. Voltage (V) 0 2 2.5 3

Capacitance (Cg) nF

Resistance (R) kX

Warburg impedance (W) S.s =2

Rct, MX

Frequency power (n)

CPE (Q) S.sn

15.77 15.75 15.28 15.42

427.2 70.32 142.2 62.9

1.171  107 … … …

… 2.46 2.19 …

… 0.5973 0.4827 …

… 2.645  108 2.129  108 …

1

manifests as current peak in I-V measurements. At 3 V, the decrease in impedance of the film manifest as rise in current in I-V measurements (Table I). For Raman spectroscopy studies, polyTHPP/IDE was biased and the spectra were measured in situ. The voltage was cycled from 0 V to 3 V with a step of 1 V. The duration of each step was 60 s, and the delay between each step was 30 s. (From here on, the voltage cycle from 0 V to 3 V will be called forward cycle and 3 V to 0 V direction called the reverse cycle.) Figure 3(b) shows Raman spectra measured at 0 V and 2 V during the voltage cycles. The spectra reveal remarkable differences in intensity and sharpness of the porphyrin skeleton bands at 1374 cm1, 1476 cm1, and 1554 cm1 between the forward and reverse cycles.17 Such decrease in peak intensity has been reported for oxidized porphyrin derivatives.17 In addition to these, the peak, at 1600 cm1, related to phenyl ring, vanishes in reverse cycle.17 Furthermore, at 0 V, a distinct increase in intensity of phenyl-peaks at 900–1000 cm1 is observed in the spectra before and after the voltage cycle. The effect of bias on phenyl moiety can be explained as electron cloud of phenyl ring is also participated in delocalization with ring electrons of porphyrin, thus oxidation of porphyrin ring affects the characteristics band of phenyl.17 Moreover, additional peaks that are observed at 1430 cm1 and 1524 cm1 indicate the lowering of symmetry of the polymer (thus the distortion of polymer structure) due to its oxidation. The changes in intensity, band structure, and the appearance of new peaks confirm the changes in the conformation of the polymer under bias. Therefore, both FRA studies and the Raman spectroscopy independently confirm the conformational change, which is related to the oxidation reduction of polyTHPP under bias.

FIG. 4. UV-vis spectra of (a) THPP solution in methanol and (b) its polymer film. (1) Molecular structure of THPP and (2) part of its polymer chain. The characteristic absorption peaks are shown on the plots.

Now, we will explain the hysteresis in Q-V measurements. The polymerization of THPP leads to change in UV-vis spectra with respect to THPP monomer; namely, absorption peaks get red-shifted (for example, 417 nm to 437 nm) and become broad (Fig. 4). These changes in spectra can be explained from the mechanism of polymerization, where meta-carbon atom of phenyl ring is involved in polymerization by ether linkage as shown in Fig. 4 (see Supplementary material for mechanism of polymerization). Therefore, the distortion of structure of THPP causes the changes in spectra; consequently, the symmetry of THPP would be affected. Since the symmetry of the THPP monomer is C2v, and the distorted C2v can have C2 or Cs symmetry, one can expect that distorted THPP might have either of these symmetry.18 Furthermore, it is known that the point group which belongs to C2 and Cs can have permanent dipole moment.18 Thus, THPP monomer having the dipole moment (4.4 D) (obtained from theoretical calculations, see Supplementary material, Sec. 1) should have permanent dipole moment even after its polymerization. Therefore, under applied electric field, the dipole moments of polymerized THPP start aligning themselves along the field direction. When the applied voltage reaches near 1 V, the THPP get oxidized and deformed (discussed above), and the oxidation would generate positive charge in the polymer matrix (the charge neutrality is maintained by counter ion incorporated during polymerization and the presence of counter was investigated by EDX, see Supplementary material, Sec. 1). Because of the presence of positive charge, the polymer matrix will be further polarized under bias. The stability of oxidized state can be envisioned from the duration of Raman spectra measurements under applied voltage, for example, the spectrum was recorded at 2 V for one minute. Moreover, the difference in current-peak-position in the I-V (at 1 V) and Z00 -peak-position in the (–) Z00 -V (at 2 V) characteristics implies that the electron-transport is faster than conformational change (that is, related to capacitance). Therefore, the restoration of distorted oxidized polymer matrix to neutral polymer lags the polarity reversal of the applied electric field. This time lag is the reason of observed hysteresis in QV and I-V. This is analogous to the cause of ferroelectric behavior in BaTiO3 which arise from the time lag between the displacement of ions and the change of electric field.19 At present, a memristor like current-voltage characteristics in ferroelectric material is thought to be the next generation low powered memory devices.19 Therefore, we investigated the memory effect of polyTHPP because of its NDR characteristics in I-V measurements. For this purpose, the high and low conduction states of the hysteresis in I-V were analyzed by applying a 10 s electrical pulse of 0 V (high state), 1.5 V (high state), 2.5 V (switching state), 1.5 V

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The research leading to this paper was funded by the State of Florida through the Florida Energy Research Consortium funds. 1

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FIG. 5. Current observed under 10 s electrical pulses of 0 V (high state), 1.5 V (high state), 2.5 V (switching state), 1.5 V (low state), 0 V (low state), 1.5 V (high state), 2.5 V (switching state), 1.5 V(low state), 0 V (low state) for 600 s, respectively. Right inset shows magnified view of the plot, and left inset shows pulse shape.

(low state), 0 V (low state),1.5 V (high state), 2.5 V (switching state), 1.5 V (low state), 0 V (low state), respectively. The corresponding current of each pulse and its magnified view are shown in Fig. 5 and at right inset in Fig. 5, respectively. The pulse shape is shown at the left inset in Fig. 5. We found that current at high conduction state was twice the current at low conduction state. These studies were carried for 50 cycles exhibiting good stability in the devices. The studies reveal that two conductions state are addressable by applying electrical pulses, even two conduction states are addressable at 0 V. Thus, poly THPP on IDE has a potential for use as low power memory devices like FeRAM.1 In conclusion, we have presented ferroelectric like behavior in electrochemically polymerized 5,10,15,20 tetra(4hydroxyphenyl)-porphyrin and shown that the behvior is related to conformational changes during redox process. The ferroeltric properties have been investigated by Q-V, I-V, and C-V measuremnets. We have shown that redox active organic materials have a potential to exhibit ferroelectric properties which can be utilized for low powered memory device. The present studies will open up an alternative route for ferroelectric properties.

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