Effect of Polyethylene Oxide on Camphor Sulfonic Acid Doped
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Synthetic Metals 257 (2019) 116176 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Effect of polyethylene oxide on camphor sulfonic acid doped polyaniline thin film field effect transistor with ionic liquid gating ⁎ Luis M. Rijosa, Anamaris Melendeza, Rolando Oyolab, Nicholas J. Pintoa, a Department of Physics and Electronics, University of Puerto Rico at Humacao, PR 00791, United States b Department of Chemistry, University of Puerto Rico at Humacao, PR 00791, United States ARTICLE INFO ABSTRACT Keywords: Field effect transistors (FET) using camphor sulfonic acid (CSA) doped polyaniline (PANi) blended with several Polyaniline polyethylene oxide (PEO) concentrations were investigated via ionic liquid gating. The pure PANi-CSA FET could Polyethylene oxide not be turned “off” and had an on/off ratio of 2. Blending with 22 wt%-PEO retained a high “on” state Ionic liquid throughput current and improved the mobility, while the on/off ratio increased by 103. Superior film quality and Transistor PEO assisted electrostatic interactions with the PANi chains led to device parameter enhancement. For higher PEO concentrations the field effect was suppressed due to disorder. Analysis of the UV/Vis spectra polaron band peak area near 810 nm show an increase in the mobility with decrease in the peak area, consistent with the observed results. Enhanced device parameters, high throughput current and low operating voltages ( ± 2 V), make PANi-CSA/PEO blends attractive materials for use in low power consumption electronics. 1. Introduction the electronics industry. Reducing the operating voltage (< 5 V) while maintaining high throughput “on” state current (> 1μA) is crucial for Conducting polymers and their blends with non-conducting poly- successful implementation in practical devices. In this paper, camphor mers play an important role in organic electronics. They are used in sulfonic acid (CSA) doped PANi thin film FET’s were fabricated with the applications ranging from metallic interconnects [1], diodes [2], pho- objective of reducing the operating voltages. Ionic liquid (IL) gating of tovoltaics [3], field effect transistors (FET) [4], chemical sensors [5], the device revealed a field effect similar to that seen in a p-type semi- solid state batteries [6,7], electromagnetic shielding [8] and rust pre- conductor. This is the first report that uses an IL to tune the con- vention of metals [9] among others. Ease of chemical synthesis, good ductivity in doped PANi. The IL was used since it has a high specific environmental stability and high conductivity has helped sustain in- capacitance which lowers the FET operating voltages ( ± 2 V) [18]. terest in conducting polymers since its discovery in 1977 [10]. Among Blending PANi-CSA with non-conducting polyethylene oxide (PEO) conducting polymers commercially available, polyaniline (PANi) has improved the charge mobility and increased the on/off ratio by three received the most attention. Depending on the oxidation level, PANi has orders of magnitude when used in a FET. Enhanced device parameters three insulating forms [11]: 1) fully oxidized pernigranaline base, 2) and low operating voltages, therefore, make PANi-CSA/PEO blends fully reduced leucoemeraldine base and 3) half oxidized-half reduced excellent candidates for use in low power consumption electronics, for emeraldine base (PANi-EB). Technologically, PANi-EB is more im- eg. pixel displays. The blend material also has additional benefits, since portant as its conductivity can be reversibly tuned by 10 orders of it retains the mechanical properties of the insulating matrix. magnitude via acid/base doping/de-doping [12]. The majority charge carriers in doped PANi are holes [13,14]. Using functionalized acids has 2. Experimental made it possible to dope PANi-EB into the conducting form and dissolve it in organic solvents [15]. The ability to process PANi in the con- 2.1. Polymer preparation ducting form has consequently led to increased conductivity that is even metallic in some instances [16]. PANi was synthesized using the oxidative polymerization of aniline The high operating voltages required in conducting polymer blend in acidic media [12,19]. In this work, PANi-EB was doped with CSA and based devices [17] are a serious drawback to their wide spread use in blended with PEO at four different concentrations. For each ⁎ Corresponding author. E-mail address: [email protected] (N.J. Pinto). https://doi.org/10.1016/j.synthmet.2019.116176 Received 5 July 2019; Received in revised form 31 August 2019; Accepted 12 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved. L.M. Rijos, et al. Synthetic Metals 257 (2019) 116176 concentration the following steps were performed: 100 mg of PANi-EB supplied VGS.VDS was swept in steps of 10 mV at a rate of 20 mV/s in all was mixed with 129 mg of CSA using a mortar and pestle, and then of the measurements. Capacitance measurements were performed in air dissolved via magnetic stirring in 10 ml of chloroform (CHCl3) for 6 h to using an Agilent Technologies 4294A impedance analyzer and a HP yield a dark green solution. The solution was filtered, and the un- 16453A modified dielectric test fixture. The film thickness was mea- dissolved solid particles dried and weighed. Based on the undissolved sured with a D-500 KLA-Tencor profilometer. UV/Vis spectroscopy was polymer mass and the initial mass, the amount of dissolved polymer in carried out using an Agilent 8453 spectrophotometer. Spectral decon- solution was determined and PEO (MW 900,000- Sigma Aldrich) was volution to determine the area were performed by first running a linear added to it at pre-selected concentrations (wt%) and dissolved via baseline correction followed by a non-linear regression using the fol- 1/2 2 magnetic stirring for an additional 2 h to yield a homogenous dark lowing function: Yfit =Y0 + (2/π) ∑(Ai/wi)exp[-2(X-Xmax, i) /wi], green solution. The following blend solutions in CHCl3 were prepared: where Y0 is an offset, Ai, wi, and Xmax,i are the peak area, peak width, 0 wt% (i.e. pure PANi-CSA), 6 wt%, 22 wt% and 33 wt%. The percola- and peak position maximum, respectively, for peaks 1 or 2 in the long tion threshold for PANi-CSA in blends with PEO was found to be just 4% wavelength region. The total peak area is the sum of the corresponding [20], where the 4% refers to the volume fraction of PANi-CSA in the areas of peak 1 and 2. The best fitting was defined by the correlation blend with PEO. Thus, in all of the blends studied here, the PANi-CSA coefficient, residuals, and the statistical parameters of the fitting vari- was above the percolation threshold. ables. All fittings were run using a combination of the Solver add-in in MS-Office 2016 and the SolvStat macro. 2.2. Ionic liquid gel preparation 3. Results and discussion The IL used in this work was prepared by dissolving 9 wt% of 1- ethyl-3-methylimidazolium bis (trifluromethylsulfonyl) amide – The PANi-CSA/PEO thin films with various PEO concentrations − (EMI)+(TFSA) (0.135 g) and 1 wt% of poly(styrene-b-methyl metha- under study were characterized by UV‐Vis spectroscopy. Fig. 2 shows crylate-b-styrene) triblock copolymer-SMS-15-81-15 (0.015 g) in 90 wt the UV‐Vis absorption spectra of the different films. Three absorption % of methylene chloride (1.35 g) to form a clear solution [18,21]. The peaks were observed for all cases. The 350 nm wavelength peak re- − anion was bis (trifluromethylsulfonyl) amide (TFSA ), while the cation presents π-π* transitions in the benzenoid/quinoid rings, at 436 nm, was 1-ethyl-3-methylimidazolium (EMI+). In order to increase its which represents the protonation (doping) on the nitrogen of imine viscosity prior to use via solvent evaporation, the IL was placed in an (low wavelength polaron band, i.e. polaron- π* transition). The peak in oven at 70 °C for several hours. the 800 nm region reflects the excitation formation in the quinoid rings (π-polaron transition) [22]. These results are consistent with previous 2.3. Device (FET) fabrication studies [23]. Two isobestic points at 450 nm and 700 nm are seen in Fig. 2 for the PEO containing films. Optical absorption spectroscopy on + Pre-patterned and doped Si /SiO2 substrates were cleaned with polyaniline reveal that isobestic points in the UV/VIS spectra indicate acetone and then with an O2 plasma etch prior to use. The pre-pat- changes in the base form of polyaniline [24]. In Fig. 2, the isobestic terned electrodes consisted of four parallel and closely spaced gold points could be due to slight changes in the oxidation state in PANi-CSA fingers. Each gold finger was connected to a large gold contact pad at via interactions with PEO. The net effect is to shift the electrical one end for hard wiring an external contact. Two adjacent fingers properties of the film away from the highly conducting form toward the eventually became the source (S) and drain (D) terminals of the device. semiconducting regime as will be demonstrated later. The presence of a The distance (L) between S/D was the channel length and W was the localized polaron band around 810 nm with no significant near-infrared channel width represented by the length of the gold finger. The sub- absorption, indicates a “compact coil” conformation of the PANi chains strates were then spun-coated (4000 rpm/45 s) with the desired PANi- [23]. A slight red shift in the high wavelength peak as the PEO wt% was CSA/PEO solution and placed in an oven at 70 °C overnight to remove increased (810 nm for 0 wt% to 820 nm for 33 wt%) suggest a charge trace amounts of the solvent.