Nazionale Bioelettronica, Marciana (LI), Italy;
¥Fondazione Elba, Rome, Italy;
+Institute of Biophysics, University of Genova, Italy.
Claudio Nicolini, Fondazione EL.B.A.,
Via Medaglie d'Oro 305 - 00136 Rome, Italy
ph: +39 6 35410728, fax: +39 6 35451637,
This is a draft paper
for a talk at the
Fifth Foresight Conference on Molecular Nanotechnology.
The final version has been submitted
for publication in the special Conference issue of Nanotechnology.
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Ultrathin films of polyanilines play an important roles for the application of this class of polymer in nanotechnology. The nature of the surface properties is critical for the function of electronic products which may be determined by the layers of nanometers thickness of polyanilines. In this regards, the Langmuir- Schaefer (LS) films of poly(o-anisidine) (POAS) substituted polyaniline have been fabricated at the molecular level. The LS films of POAS conducting polymer were investigated by using Brewster microscopy, ellipsometric and electrochemical techniques, respectively. The obtained results revealed a regular deposition during at least 40 monolayers of POAS LS films. The development of the surface irregularities beyond 40 monolayers of LS films resulted into an electrochemical kinetics similar to the electrodeposited films. The electrochemical kinetics in fewer number of monolayers was shown to be the first electron transfer process. The nature of anions caused significant changes in the redox properties of POAS LS films. The electrochromic switching response time and diffusion coefficient of such LS films were also estimated by electrochemical investigations. Further, POAS LS films were used for the detection of HCl in water by conductrimmetric measurement, which revelaed a detection limit of 0.1 ppm of HCl.
Polyaniline class of conducting polymer has been widely investigated aiming towards potential applications for rechargeable batteries, protection coating, electrochromic displays, conducting composite material, gas sensor, corrosion protection, electronic, biosensors and optical devices (1-6). In fact polyaniline often has been categorized as an intractable polymer due to the lower processibility (7). In this regard, efforts have been made in increasing the processibility by using the subtitutents groups in the monomer or the polymeric backbone (8-10). Substituted polyanilines such as poly(ortho-toluidine), poly(methoxy aniline) (POAS), poly(ethoxy aniline) (PEOA) and poly(methyl aniline) etc. have been synthesized keeping in view the unusual electroactive properties and improved processibility (11-12). The substituted polyaniline increases the possibilities of adopting various techniques in the fabrication of conducting polymer from thick to ultra thin films, which are appropriate for several technologies as well as electronic and optical devices (13-14). In the recent past such ultra thin films of substituted polyanilines were fabricated by Langmuir-Blodgett (LB) technique aiming towards its potential application in nanotechnology (15).
In fact, the recent past had also shown the deposition of quasi -ordered and ordered Langmuir- Blodgett films of polyaniline, where emeraldine base dissolved in N-methyl 2-pyrrolidinone (NMP) /chloroform (CHCl3) could be used to cast LB films using an aqueous subphase containing neutral or acidic water (16-17). The deposited LB films of polyaniline used to give rise the irregular surface after certain number of deposited films. The ultra thin films of POAS and PEOA have been recently fabricated by Langmuir-Blodgett technique (15, 17). POAS LB films can be considered to be an important class due to their optical and electrical properties (16). The conductivity and optical properties of POAS depends upon the oxidation state of the main chain as well as the degree of protonation of nitrogen atoms in the polymer backbone. The doping brings the different structure from emeraldine base to emeraldine salt (16). So, it is an important factor in knowing about the organization of the polymer molecules at the air/water interface as well as the physical properties of the deposited films.
Beginning with this consideration the aim of this paper was to discuss the various parameters ruling the quality of Langmuir film as well as optical, electrochemical and structural properties of POAS LB films. The POAS LB films were studied at the air/water interface by Brewster angle microscopy (BAM). Additionally, the deposited films were investigated using cyclic voltammetry in order to assess the effort of redox states for the surface morphology in going from thin to thick films. The electrochromic switching response time at different protonic acid media was studied on such POAS LS films. The possibility of using such POAS LS films for the HCl sensor were also performed, which showed a detection limit of the 0.1 ppm.
The poly(ortho-anisidine) (POAS) was oxidatively polymerized using ammonium peroxy-disulphate as reported previously (16, 18). The emeraldine base (EB) powder was used for the fabrication of the Langmuir-Schaefer (LS) films. A solution was prepared by dissolving 1mg of POAS (EB) conducting polymer in 5 ml of CHCl3. The resulting solution was filtered through the resistant filter (0.5 m). LB trough [MDT corp.] of size 240 mm x 100 mm and volume 300 ml was used for the fabrication of LS films with a compression speed maintained at 0.2 mm/sec. The Langmuir trough was filled with an aqueous subphase containing pH 1 using HCl acid. 70 l of the solution was spread on a acidic subphase. Different numbers of monolayers were transferred on to glass indium-tin-oxide (ITO) plates and  silicon substrates by Langmuir-Schaefer (LS) method at a pressure of 25 mN/m with a feed back of 5mN/m.
Ellipsometric measurements were performed using a PCSA null ellipsometer LEPh-2 (Special Design and Production Bureau for Scientific Devices of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk) using He-Ne laser (wavelength 632.8 nm). The accuracy of the device was 0.02° with respect to and . The POAS LS films deposited on  silicon substrates attributed anisotropy, we kept an ideal condition of ellipsoid position at 70° by properly optimizing our system before the measurement of the LS films. Various monolayers (from 1 to 60) of POAS LS films were recorded in the similar condition. The thickness of the LS films were calculated by using the two layer model (19).
The UV-visible spectra of POAS LS films deposited on quartz substrates was recorded by using the UV-visible spectrophotometer (Jasco model 7800).
The electrochemical measurements were made by Potentiostat/Galvanostat (EG & G PARC model 163 with a software M270). A standard three electrodes configuration were used, where LS films on glass ITO plate worked as a working electrode, platinum as a counter and Ag/AgCl as a reference electrode. The cyclic voltammogram was measured at different scan rate of POAS LS films of POAS at different concentrations of protonic acid (HCl).
The formation of Langmuir films of POAS, as well as their deposition onto a glass plates, was analyzed by Brewster Angle Microscopy (BAM), in particular with a BAM2 microscope (Nanofilm Technologie Gmbh, Göttigen, Germany) coupled with a NIMA LB trough (NIMA, England, type 601; surface pressure sensor type PS3). BAM images of POAS Langmuir film were acquired during its formation and after its deposition on quartz plates. The analysis allowed one to study the morphological features of such films as well as to reveal the 2D-3D transformations which was taken place at different surface deposition pressures (20).
The stability of Langmuir monolayer is somewhat associated to a high collapse pressure, a steep increase in the surface pressure curve in the condensed phase and a small hysteresis in the compression-expansion cycle. The choice of the appropriate subphase has been proved extremely important for the deposition of LB film from polyanilines films (16). Fig.1 (curve 1) shows the pressure -area isotherm for the POAS Langmuir film at pH 6.4. It does not show a steep increase of pressure in the condensed phase and the yielding (breaking) point for the pressure has been obtained at a pressure of 42 mN/m. It attributes that higher pH does not produce good Langmuir monolayers. So a careful investigation was performed using different pH of the sub-phases. When the pH is maintained either 5 (curve 2) or 4 (curve 3), no steep increase in the pressure-area isotherm behavior is obtained. But, when pH of the medium is lowered (curve 4 to 6) the molecules suffered and are doped where Langmuir monolayer is more crystalline in nature. The Langmuir monolayer at lower pH (1 to 3) gives rise to higher collapse pressure ( 50 mN/m) than higher pH (4 to 6.4). The importance of doping has been confirmed for the stable monolayers, which is probably associated with the ordering that is introduced in the polymer. It can be speculated that the HCl doping organizes the polymer chain of POAS Langmuir film. The solid phase transition has started occurring at the molecular area of 6 Å2 between 1 to 3 pH. So an attempt was made for the estimation of molecular area of POAS at air - water interface by extrapolating the pressure -area isotherm curve at different pH of the subphase. It shows that when the pH is maintained 6.4, the molecular area has been estimated to be 55 Å2. For the calculation of molecular area at condensed phase, one repeating unit of POAS has been taken into account (16). When the POAS monolayer is formed at lower pH, the polymer chain orients and doped with protonic acid. So the area obtained at 1 pH has been found to be about 21 Å2 which should be the expected area of the aniline monomer. The cross sectional area of the aniline repeat unit at air - water interface has already estimated to be 20 Å2. So, 25 mN/m pressure has been carefully selected to deposit the LB films at a subphase pH 1.
Fig.1 Variation of the pressure vs area isotherm at different pH using HCl acid
The formation of Langmuir films of POAS, as well as their deposition onto a quartz plates, was analyzed by Brewster Angle Microscopy (BAM), in particular with a BAM2 microscope (Nanofilm Technologie Gmbh, Göttigen, Germany) coupled with a NIMA LB trough (NIMA, England, type 601; surface pressure sensor type PS3). The working principle of BAM is based on Fresnel's principle for polarized light from an interface and it is described in literature (21). BAM images of POAS Langmuir film were acquired during its formation and after its deposition. The analysis allowed one to study the morphological features of such films as well as to reveal the 2D-3D transformations which was taken place at different surface deposition pressures. The POAS Langmuir film formation was analyzed by means of BAM for both ES forms, which were deposited at pH 1. 2D and 3D transformations were imaged and correlated registered with the simultaneously p/A isotherm. It was assessed that the dopant arranged the POAS molecules at the air/water interface in a different way, as underlined by the decrease of area per repeating unit (r.u.). This fact underlined that the inclusion of dopant ions in the molecule, from EB to ES form of POAS polymer, was able to cause structural and conformational changes due to a different charge distribution in the backbone. Moreover, the BAM analysis allowed one to reveal the presence of breaks, aggregations and collapses. These features represent a crucial point to be considered during the design of devices based on highly ordered layered structures with low concentration of defects. A note fact is that part of such defects are present in the deposited films which is also already seen at the air/water interface as shown in Figure 2. Breaks revealed at the interface (Figure 2 image A) were observed in a layered structure of POAS (Figure 2 - image C). Furthermore, the images B and D of figure 2 underlined that the 3D transformations which was taken place reaching the 2D limiting molecular density, namely collapses (Figure image B), were a irreversible phenomenon, as confirmed by the presence of such 3D structures on deposited POAS films. Breaks (image A) and collapses (image C) formed at the air/water were revealed also on deposited films of POAS (images B and D) thus suggesting that such defects were already present at the air/water interface. A layered structure of POAS Langmuir layer is displayed in image B: the different gray levels represent the overlapping of monolayers.
[Full size Figure 2: 58K, 645 x 683 pixels]
Fig.2 Brewster angle microscopy: Image A (monolayer at air/water interface at 25 mN/m), Image B (monolayer at air/water interface at 45 mN/m), Image C (monolayer at quartz plate deposited at 25 mN/m) and Image D (monolayer deposited on quartz plate at 45 mN/m).
The thickness of the deposited POAS LB films at a pressure of 25 mN/m was estimated by ellipsometric measurements. Fig.3 shows the thickness (estimated from ellipsometric measurements) vs the number of monolayers of POAS LS films. It reveals a linear fitting measured till 36 monolayers of the POAS LS films. The thickness obtained for each monolayer was estimated to be 24 2 Å, which resembled with the thickness measured by x-ray diffraction (16, 19). It attributes that the each deposited monolayer of POAS molecules are parallel to the surface (substrate) while measured till 36 monolayers. Whereas, it attributed a decrease in the thickness of one monolayer layer calculated from the LS films containing 40 monolayers. Such decrease in the thickness of each monolayer (after 40 monolayers) may perhaps be linked to the less transfer of the Langmuir monolayer molecules or some molecules of POAS are blown in process of drying under the nitrogen flux. We see the deviation in linearity for the films deposited after 36-40 monolayers in Fig.3. So, an attempt was taken to present the two different slopes for the films deposited till 50 monolayers of POAS LS films.
[Full size Figure 3: 5K, 638 x 355 pixels]
Fig.3 Variation of thickness vs number of monolayers of POAS LS films measured by ellipsometric measurements.
Fig.4 (inset) shows the UV- visible spectra of 20 monolayers of POAS LS films on quartz substrate. It reveals to the two sharp absorption bands at 348 nm and 750 nm of the films made by using 1 pH aqueous subphase. The observed peak at 348 nm attributes to a -* transition centered on the benzoid ring (inter band transition) and the band seen at 750 nm is due to the HCl incorporation, when emeraldine base form of POAS monolayer is formed at pH 1 containing acidic (HCl) subphase (22). In our earlier work, it was shown that 1 M HCl doping caused the appearance of 840 nm and 450 nm bands besides the shift of the bands from 348 nm to 352 nm. The uniformity in deposition of POAS LS films has also been characterized using UV-visible absorption spectra. The magnitude of UV-visible absorption band at 348 nm for number of monolayers is shown in Fig.4. It shows for the increment in absorption magnitude from one to sixty monolayers of POAS LS films. Interestingly a linear increase of the UV-visible absorption magnitude till 37 monolayers can be observed. The deviation in the linearity in the UV-visible absorption magnitude can be observed nearly at around 40 monolayers of LS films. So, attempts were made in showing the two different slope of the number exceeding than 37 monolayers, which resembles to the ellipsometric measurements.
Fig.4 Variation of UV visible absorbance at 348 nm vs number of monolayers. Inset show the UV -visible spectra for the 20 monolayers POAS LS films made at pH 1.
The little change that occurred in drawing the number of layers in ellipsometric and UV-visible absorption measurements may be arising due to the fact that the films were deposited on the two different substrates (silicon and quartz crystal). The nature of substrates strongly influences the deposition parameters of Langmuir monolayers. The above techniques indicate for the beginning of irregular deposition beyond 40 monolayers.
The electrochemical kinetics of various deposited monolayers of POAS LS films have been studied by performing cyclic voltammetry in a electrochemical cell consisting of three electrodes in 1 M HCl medium at a scan rate of 50 mV/sec. Inset in Fig.5 shows the electrochemical response of one monolayer of POAS LS film. The first oxidation converts the film from neutral to polarons states and second oxidation leads to states of bipolarons. Since the second potential involves the more electrons than first, we therefore, cannot see the same peak potential height in one monolayer of POAS LS film. The CVs of 1 monolayer to 60 monolayers of POAS LS films are shown in Fig.5. The shape of the cyclic voltammogram for each number of layers shows the surface confined species as expected. The cyclic voltammetry peaks are associated to the oxidation and reduction processes of POAS LS films. The peak current scale increases linearly as a function of monolayers as shown in Fig.5. It shows the little change in the value of peaks potential in going from monolayer to multilayers deposition. In fact, this shift in the peak potential can be related to the increase in the more polarons/bipolarons states for the number of monolayers of POAS LS films. The oxidation and reduction peaks potential of 10 monolayers in Fig.5 (curve 3) have the reduction peaks at around 724, 436, 186 and -4 mV, whereas the oxidation peaks can be seen at 724, 496 and 282 mV. It can be attributed that there is a little shift in the oxidation potential at 720 mV, whereas there is gradual increase in the reduction potential at 720 mV for the number of monolayers. The electrochemical response of POAS was practically same as a function of number of monolayers, besides the oxidation couple at around 724 mV is affected by changing the number of monolayers (23). In addition the lower value of electrochemical reduction potential (at 720 mV) of POAS LS films can also be noticed in close comparison to the polyaniline films (at 0.8V). The change in reduction potential value is linked to the higher electronic density states due to the substitution in the aromatic ring, which facilitates the protonation and the oxidation of the amine group. Curve 8 in Fig.5 shows the electrochemical response of 60 monolayers. It shows the quenching of the peak at 720-740 mV as well as the broadening and decrease in the intensity of the peak potential at round -20 to 0 mV, which may perhaps be linked to the irregular deposition and the increase in film thickness. The widening of the peak potential at 720 mV were also observed while going from 44 to 60 monolayers. It could also be related to the inhomogeneity incorporated inside the film in the process of deposition process beyond 40 monolayers. However, it is worth drawing attention to the fact that the electroactivity of the polymer depends on the film thickness. The similar results were observed for the electrochemical grown polyaniline films having different thicknesses. The electroactvity is higher in the thin LS films and decreases in rising the thickness. The widening in the peak potential value from 44 to 60 monolayers can be seen for the POAS LS films. The resulted cyclic voltammetric for the lower number of LS films are linked to more number of oxidation and reduction peak potentials and faster electrochemical response, whereas CV of the films containing higher number of monolayers behave little differently in the electrochemical process. The film formed by using the pulse potential (potentiodynamic) shows a similar behavior to the LS films of sixty monolayers. The redox behavior of POAS deposited electrochemically was well established by Mello et al (23), including such aspect as the dependence of pH of the solution and relation to the method of preparation (23). Though, it may seem analogous that electrochemically prepared films thicker than multilayer LS films, are also regular (23). The key factor that determines CV response is surface regularity. The slowing down of electron transfer process was also seen by evaluating the half peak potential vs the number of layers, which shows the irregular deposition beyond 40 monolayers as observed by UV-absorption and ellipsometric techniques, respectively
[Full size Figure 5: 11K, 623 x 444 pixels]
Fig.5 Cyclic voltammogram (CV) of POAS LS films as a function of monolayers viz.: 1 = monolayer, 2 = 5 layers, 3 = 10 layers, 4 = 15 layers, 5 = 25 layers, 6 = 34 layers, 7 = 44 monolayers, 8 = 60 layers; Inset shows the CV of one monolayer of POAS LS film.
In order to verify, the oxidation and reduction processes of POAS conducting polymer films and diffusion coefficients, the CVs of 40 monolayers of POAS LS films were recorded at different scanned rate. The observed CV response in Fig.6 underlying to the notion that the redox kinetics is probably controlled by ohmic effect. The gradual change in the peak potential have been observed, quite similar to the electrodeposited films. The value of diffusion coefficient (Do) in different media for POAS LS films was determined using the Randles -Sevics equation (24, 25). The diffusion coefficient (Do) has been calculated 1.2 x 10-10 cm2s-1.
[Full size Figure 6: 7K, 638 x 355 pixels]
Fig.6 CVs of 20 monolayers LS films at different scan rate as shown in Figure.
The other interesting aspect such as electrochromic effects of the films were also observed. The electrochromism was noted from yellow to green and later to violet as the potential was swept from -0.2 to 1.0 V for such POAS LS films. So the oxidation and reduction processes in POAS LS films were studied in strong (HCl) to weak acid (acetic acid) as shown in Figure 7. These changes are associated to the variations in the electronic resonant structure of the polymer backbone caused by oxidation and protonation forms of POAS conducting polymer. The overall process involves the loss of two electrons and deprotonation for POAS conducting polymer The oxidation and reduction process and the lower redox switching of the POAS conducting polymer may be dependent on redox ionic conductivity of the polymer matrix. It has been shown that HCl ions show the faster electrochromic effect (switching half time Hf1/2= 250 ms) than studied in H2SO4 (Hf1/2= 300 ms) and CH3COOH (Hf1/2 = 1 sec). The oxidation and reduction process in CH3COOH medium is slower than studied either in H2SO4 or HCl media (26). The weak acid takes time in diffuses to the POAS films.
Fig.7 Oxidation and reduction current response for 30 layers the POAS LS films Viz.:(a) HCl, (b) H2SO4 and (c) CH3COOH media.
The HCl sensing in water was performed by depositing 40 monolayers of POAS LB films on the interdigited electrode and the films was undoped in aqueous ammonia for the five minutes. The films was washed in water and dried by blowing nitrogen gas. Such POAS LS films was subsequently dipped at a concentration of HCl solution for five minutes and current -voltage measurement was performed. Later, the films was dipped in the higher concentration for the same time period, dried and I-V characteristics was measured. This procedure was repeated for repeated for the higher concentration. The magnitude of the current measured at 0.5 V was given in Figure 8. Significant changes in the properties of POAS are observed as the amount of HCl ions in the solution increases. There is a continuos increase in the current vs HCl concentration. The presence of small amount of HCl ions diffuses in to the films and which can be sensed.
[Full size Figure 8: 5K, 626 x 369 pixels]
Fig.8 Response curve as current vs HCl concentration.
In summary, we have indicated the uniformity of POAS Langmuir-Schaefer films are restricted to certain number of monolayers. The thickness of one monolayer estimated from 1 to 40 monolayers is to be 24 , whereas it shows a decrease in thickness of monolayer measured from 40 to 60 monolayers. It can be noted that the non-linearity in the ellipsometric or UV-absorption measurements put forward for the real deposition process. However, irregularities begin to form after 40 monolayers and the orderliness in the film decreases, leading to the electrochemical kinetics of the electrochemical deposited films. The electrochromism has been studied in the POAS LB films, which shows a good colour contrast in 40 layers of the films. The HCl sensor shows the detection limit of 0.1 ppm.
We are thankful to Drs. Victor Erokhin, M. Adami, M. Sartore and M. Salerno for their interesting discussions during the preparation of the manuscript. Thanks are due to Mr. F. Nozza and Miss Rando for their help in carrying out the experiments. Financial supports from EL. B.A. Foundation and the University of Genoa are gratefully acknowledged.
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