Infrared Optics Applications of the Thin Polyaniline Emeraldine Base Films

PREPARATION OF SE-DOPED POLYANILINE EMERALDINE BASE FILMS Edward Bormashenko*a, Roman Pogreba, Semion Sutovskia, Alexander Shulzingera, Avigdor Sheshneva, Alexander Gladkikhb aThe College of Judea and Samaria, Ariel, 44837, Israel bWolfson Applied Materials Research Center, Tel Aviv University, Tel Aviv, 69978

Abstract Our letter presents the possibility to produce polyaniline emeraldine base films doped with selenium. PANI EB and Se were contemporaneously dissolved in dimethylsulfoxide. Thin films were obtained by spin-coating of Si and ZnSe substrates with a PANI EB + Se solution in DMSO. We studied the distribution of the selenium in the PANI EB films using TOF-SIMS spectrometry. IR spectra of obtained films were investigated with a FTIR spectrometer. Keywords: Polyaniline, selenium, dimethylsufoxide, doping, TOF-SIMS, infrared, spectrum.

*Corresponding author E-Mail: [email protected]

4 - 21 1. Introduction Polymers containing selenium have been the subject of much investigation recently, due to their promising properties as organic superconductors [1-2]. Tetraselenafulfalene and other selenium-containing organic compounds have displayed metal-to-semiconductor (insulator) transition at low temperatures. PANI derivatives doped with selenium may be of interest both from the point of view of understanding the fundamental processes, and with a view to possible applications. However obtaining of such compounds was not discussed yet. The essentials of doping of PANI and PANI derivatives were explained by Heeger, MacDiarmida and Epstein [3-4]. The study of doping PANI with non-organic dopants such as palladium and platinum compounds is still in intensive progress [5- 6]. The use of different metal compounds: TiO2, FeCl4, vanadium, tungsten and molybdic containing acids as dopants has been the subject of much investigation recently [7-8]. IR spectra of PANI derivatives and influence of dopants on the IR absorption spectrum of PANI was studied using Raman and FTIR spectroscopy [6, 9- 13]. The infrared spectrum of selenium was studied by R.B. Stephens [14]. The infrared spectra of selenium-containing organic compounds were investigated recently [15]. It was shown by Gal and Hodes that Se demonstrates solubility in DMSO [16]. This fact allows the preparation of Se-doped PANI EB films. 2. Experimental The polyaniline emeraldine base and dimethylsulfoxide were supplied by Sigma-Aldrich Israel Ltd. ZnSe plates were supplied by Eksma Ltd (parameters of the plates: transmission band 0.6-22 μm, refractive index in the band 8-13 μm, n = 2.417- 2.385). Se in the form of irregularly shaped pellets with a diameter of 2-3 mm (purity > 99. 995%) was supplied by Sigma-Aldrich Israel LTD. First 0.1% solutions of PANI EB in DMSO were prepared, then Se pellets were added to the solution and stirred under t = 60 ºC for 24 hours. The Se was present in excess in the solution; Gal and Hodes noted that dissolved Se concentration in DMSO equals 5 mM [16]. Se is distinguished by its toxic properties and DMSO is readily absorbed through the skin, so care should be taken when working with this solution. Then Si and ZnSe substrates were coated with the obtained solutions by the spin-casting process and exposed to vacuum evaporation. The absorbance spectra of coated with PANI EB and Se-doped PANI EB ZnSe substrates in the middle and far IR bands (400-7500 cm-1) were established using a Bruker Vector 22 FTIR spectrometer. Coated Si substrates were studied by time-of-flight secondary ion mass spectrometry (TOF-SIMS), using a TRIFT-2 mass-spectrometer produced by Physical Electronics Co. 3. Results and discussion Si substrates were coated with the prepared solution of PANI EB and Se in DMSO (films with a thickness of 100-400 nm were obtained) and studied by TOF- SIMS spectrometry, which indicated the presence of Se in PANI EB films (see Fig. 1- 3). Concentration of selenium in the surface layer was estimated as 5·1011 atoms/cm2. Depth scanning of the films indicated homogeneous distribution of Se in the PANI EB films (Fig. 2). Surface mapping (Fig. 3) of the samples revealed that as well as being distributed uniformly in the film, Se forms clusters with an average dimension of ~ 10 μm. Concentration of selenium in such clusters is one order of the magnitude higher than the average concentration of selenium in the PANI EB films. The concentration of Se in obtained PANI EB films was relatively small (estimated roughly as 1-10 ppm), so significant changes in the IR spectra could not be

4 - 22 anticipated. Nevertheless we studied the IR spectra of the Se-doped PANI EB films. ZnSe substrates were spin-coated with a SE+PANI EB solution in DMSO and exposed to vacuum evaporation. Vacuum evaporation of the ZnSe substrates coated with Se-doped PANI EB films was carried out using an optical bench, which allowed “real time” control of the process. The bench is described in Fig. 4. Absorption spectra of the PANI EB coating were taken when vacuumed using a FTIR spectrometer Bruker Vector 22. Evaporation of the solvent was accompanied by changes in the absorption spectra. The termination of such change is indicative of the net removal of the solvent from the PANI EB film. We established that middle IR spectra of pure PANI EB and Se-doped PANI EB are practically identical (see Fig. 5). We didn’t observe splitting or displacement of PANI EB peaks described by Quillard and Macdiarmid [13]. This fact can be explained easily: Selenium and selenium-containing organic compounds demonstrate absorption in far infrared band only [14, 15]. Se-Se stretching mode is characterized by absorption at 285 cm-1, and Se-C stretching vibration demonstrates absorption at 452 cm-1. We really revealed the new peak inherent for the Se-doped PANI EB films at 872 cm-1 (see Fig. 6). Pure PANI EB doesn’t demonstrate absorption at this frequency. It is reasonable to suggest that frequency 872 cm-1 is overtone of Se-Se stretching vibration, which demonstrate main absorption peak at 285 cm-1. Exhaustive information about IR absorption spectra and chemical structure of the Se-doped PANI films could be obtained using spectrometric equipment, which provides spectral data in far IR band (400 cm-1-200 cm-1). We concentrate our efforts on the study of far IR spectra of the Se-doped PANI EB films now.

4. Conclusions PANI EB films doped with Se were obtained. The distribution of the dopant in the PANI EB films was studied using time-of-flight secondary ion mass spectrometry. The IR spectra of the SE-doped PANI EB films were investigated with FTIR spectroscopy. Acknowledgements This work was supported by the Israel Ministry of Science, Culture and Sport (Project No. 1461-2-00) and the Israel Ministry of Absorption. The authors are thankful to Professor Alexander Voronel for his generous support of our experimental activity and fruitful discussions.

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4 - 23 [11] A. Dhanabalan, S.S. Twalar, A.Q. Contractor, N.P. Kumar, S.N. Narang, S.S. Major, K.P. Muthe, J.C. Vyas, Journal of Material Science Letters, 18 (8), (1999), 63. [12] M.C. Bernard, T.B. Vu, S. Cordoba-de-Torresi, A. Hugot-Le-Goff, Proceedings of SPIE, 3145, (1997), 376. [13] S. Quillard, G. Louarn, S. Lefrant, A.G. Macdiarmid, Physical Review B, 50 (17), (1994), 12 496. [14] R.B. Stephens, Journal of Applied Physics, 49 (12), (1978), 5855. [15] Z.V. Popovic, V.A. Ivanov, O.P. Khoung, T. Nakamura, G. Saito, V.V. Moshchalkov , Synthetic Metals, 124, (2001), 421. [16] D. Gal, G. Hodes, Journal of the Electrochemical Society, 147 (5), (2000), 1825.

4 - 24 31UNSAVED - Ions 100µm 4579213 cts

) 80000 79.91 n i b

m 75.96 a

60000 1 0 . 0 (

t 40000 n

u 77.91 80.90 o 78.91 C

l 81.94 a t 20000 76.96 o T

0 74 76 78 80 82

31UNSAVED - Ions 100µm 4579213 cts 14000 ) n i 12000 b u 79.906 m 10000 a

0 8000 . 0 ( s t 6000 n u o

C 4000 l a t o 2000 79.961 T

0 79.6 79.8 80.0 80.2 80.4

Fig. 1. The part of the mass spectrum obtained on the PANI EB film containing Se (upper graph) and representing all isotopes of Se (74Se, 76Se, 77Se, 78Se, 80Se, 82Se). The lower graph is enlarged image of 80Se mass peak (most intensive).

4 - 25 ï ì Si 105

104 Se s t n 103 u o C

100 0 20 40 60 80 100 Time (Seconds)

Fig. 2. Shallow depth profile of the Se doped PANI EB film deposited onto Si substrate. The part of the profile (from 25 s to 50 s of sputter time) was used for calculation of the surface concentration of Se in the sample, which was estimated as 5·1011 atoms/cm2.

4 - 26 Fig. 3. TOF-SIMS image in Se negative ions of the Se- doped PANI EB films (light shapes – clusters of Se, dispersed in PANI EB matrix)

4 - 27 Fig. 4. Optical bench used for controlling vacuum evaporation of Se-doped PANI EB films

4 - 28 Fig. 5. Infrared absorbance spectrum (absorbance, a.u. vs. wavenumber) in 1100-1600 cm-1 of (1) PANI emeraldine base and (2) Se –doped PANI emeraldine base

4 - 29 Fig. 6. Infrared absorbance spectrum (absorbance, a.u. vs. wavenumber) in 800-900 cm-1 of (1) PANI emeraldine base and (2) Se –doped PANI emeraldine base

4 - 30