pubs.acs.org/Langmuir © 2009 American Chemical Society
Doping-Induced Conductivity Transitions in Molecular Layers of Polyaniline: Optical Studies of Electronic State Changes L. Cristofolini,*,† M.P. Fontana,† P. Camorani,† T. Berzina,† and A. Nabok‡ †
Dipartimento di Fisica, INFM-CNR CRS-Soft, Universit a di Parma, Parma, Italy and Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield, U.K.
‡
Received October 5, 2009. Revised Manuscript Received November 23, 2009 The doping-induced conductivity transitions in molecular layers of polyaniline have been studied by monitoring the correlated optical and spectroscopic changes using spectroscopic and single wavelength extinction ellipsometry, also in total internal reflection mode (TIRE), together with reflection spectrometry. The measurements were performed on deposited multilayers as well as on a Langmuir monolayer at the air-water interface, as a function of acidic doping. We found that the characteristic spectroscopic features of conducting and insulating polyaniline persisted down to the single layer, both in the solid state and at the air-water interface. We also investigated in real time the modulation of conductivity induced by the intercalation of Li ions in the polyaniline film, by a combination of time-resolved ellipsometry and reflectivity spectra measurements. In this case, the enhanced sensitivity provided by the TIRE geometry, combined with the relatively fast time scale accessible by the single wavelength ellipsometry, allowed us to follow in detail in real time the doping/dedoping process.
1. Introduction Among the several interesting properties of conducting polymers, the actual mechanism underlying their electrical behavior and the conductivity transition has attracted much interest.1-6 In as much as these materials in their high conductivity state can be considered as metals, they can provide a useful and somewhat unconventional system to study the metal-insulator transition7 in low dimensional disordered systems.8 Recently in fact the actual metallic character of well prepared high conductivity thin films of polyaniline (PANI) has been clearly demonstrated.9 The thickness dependence of the conductivity has also been studied;10 however, the quality of the films (prepared electrochemically) was not sufficiently high to allow the clear extraction of the influence of thickness on the observed behavior. To our knowledge little information is available on the conductivity behavior as a function of thickness in high quality (e.g., LangmuirBlodgett (LB)) multilayers, where the actual role of disorder on the occurrence of the doping induced conductivity might be evinced more clearly and separated from the eventual thickness dependence. Therefore, an interesting question concerns the possibility of metallic behavior in the Langmuir monolayer at the air-water interface itself; according to conventional wisdom,8 such behavior should not be observable in the very peculiar morphology of the 2D Langmuir film. Recently, we have reported a microscopic study of structural and morphological modifications in PANI mono and multilayers
following doping by X-ray reflectivity, grazing incidence diffraction and X-ray induced fluorescence. DC conductivity data were also taken on the deposited multilayers.11 In this paper;which can be considered as a follow-up;we bring additional evidence obtained by a study of the complex dielectric response function in high quality multilayers of the prototype conducting polymer PANI, as a function of doping level and for different layer thicknesses, down to the single deposited monolayer at the solid-water interface and in the floating monolayer at air-water interface. PANI monolayers on the air-water interface were fully characterized by the study of Langmuir isotherms; spectroscopic and single wavelength null ellipsometry were used to study the deposited multilayers and the monolayers at the air-water interface as a function of doping level, hence in the conducting and insulating states. In particular, we have made full use of the large sensitivity increase of a recently reported modification of the ellipsometric technique (total internal reflection ellipsometry) which involves surface plasmon resonance enhancement.12-14 The measurements reported here confirm on the frequency scale of electronic processes in the visible and near IR the main conclusions of our previous paper, i.e., that in properly prepared high quality PANI films metallic behavior persists down to the single monolayer. Given the relative ease of modulating the electrical conductivity by controlling the redox state of PANI thought ionic doping, these results indicate this system for further studies of metal-insulator transitions of complex materials in confined geometries.
*Corresponding author. (1) MacDiarmid, A. G. Curr. Appl. Phys. 2001, 1, 269–279. (2) Heeger, A. J. Rev. Mod. Phys. 2001, 73, 681–700. (3) Heeger, A. J. Phys. Scr. 2002, T102, 30–35. (4) Tzamalis, G.; Zaidi, N. A.; Homes, C. C.; Monkman, A. P. Phys. Rev. B 2002, 66, 085202. (5) Lee, K.; Heeger, A. J. Synth. Met. 2002, 128, 279–282. (6) Kaiser, A. B. Adv. Mater. 2001, 13, 927–941. (7) Mott, N. F. Metal-insulator transitions; Taylor & Francis: London, 1990; (8) Phillips, P.; Dalidovich, D. Science 2003, 302, 243–247. (9) Lee, K.; Cho, S.; Park, S. H.; Heeger, A. J.; Lee, C. W.; Lee, S. H. Nature 2006, 441, 65–68. (10) Al-Attar, H. A.; Al-Alawina, Q. H.; Monkman, A. P. Thin Solid Films 2003, 429, 286–294.
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2. Experimental Section Polyaniline emeraldine base (PANI) was purchased from Aldrich, (average Mw ca. 100 000, catalogue no. 576379). Polyaniline powder first was dissolved in 1-methyl-2-pyrrolidone (NMP)
(11) Cristofolini, L.; Fontana, M. P.; Konovalov, O.; Berzina, T.; Smerieri, A. Langmuir 2009, 25, 12429–12434. (12) Arwin, H. Phys. Stat. Sol. A 2001, 188, 1331–1338. (13) Arwin, H.; Poksinski, M.; Johansen, K. Appl. Opt. 2004, 43, 3028–3036. (14) Nabok, A.; Tsargorodskaya, A. Thin Solid Films 2008, 516, 8993–9001.
Published on Web 12/15/2009
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Article in the concentration of ∼1 mg/mL (mother solution). Then NMP with 10% of toluene was added to some amount of mother solution to obtain the final concentration of 0.1 mg/mL and filtered through Millipore filter (Omnipore PTFE, hydrophilic, pore diameter 0.2 μm). Toluene was added to facilitate solution spreading. This solution was used for the preparation of the Langmuir monolayer and Langmuir-Schaefer films as described previously11. Langmuir isotherms were routinely obtained in a KSV 5000 trough or in our homemade trough both to check the monolayer quality for deposition and to prepare the monolayer at the air-water interface at the proper surface pressure for the measurements. Samples with a variable number of layers (from 1 to 44) were deposited by the horizontal lifting, or LangmuirSchaefer (LS) technique. As substrates we employed BK7 glasses preliminary coated by typically 19 nm thick evaporated gold film. Each substrate was individually characterized by ellipsometry and AFM prior to the PANI film deposition to get the exact thickness of gold film (within 0.05 nm) and its typical roughness. This sample geometry allows ellipsometric measurements both in the conventional “external reflection” mode, and in the socalled total internal reflection mode ellipsometry (TIRE),12-14 a very sensitive tool capable of reliable detection, for instance, of submonolayer coverage of a protein layer at a solid/liquid interface and also to resolve details in the kinetics of layer formation, with the important advantage of the “internal reflection” based ellipsometry: the possibility to analyze turbid solutions. Moreover, in comparison with the more common surface plasmon resonance technique, in TIRE the simultaneous measurement of two ellipsometric parameters Ψ and Δ, corresponding respectively to the ratio of the intensities and the phase shift of “p” and “s” components of the reflected light, yields additional and useful information for quantitative analysis and for the determination of the experimental parameters of the adsorbed layer. However, to the best of our knowledge, all the TIRE experiments reported in the literature rely on a spectral analysis, i.e., are performed with a spectroscopic ellipsometer. In the present work we also employed the same TIRE geometry in conjunction with a highly sensitive monochromatic null-ellipsometer and achieved an enhancement of its sensitivity comparable with that obtained in the more common “spectroscopic” TIRE; this enabled us to follow more precisely the kinetics of the transformation down to the level of the single monolayers. Measurements were performed either in extinction mode at the single wavelength λ = 633 nm with the Multiskop (Optrel GbR) ellipsometer operating in the PCSA configuration, or in spectroscopic mode using the M2000 V (J.A. Woollam) ellipsometer operating in the range of 370-1000 nm. The incidence angle of 45 was chosen for the measurements on PANI layers on the water surface; the incidence angle of 68 for the TIRE measurements on the glass-water interface was provided by a 68 trapezoidal BK7 glass prism. TIRE measurements were performed either with the PANI film in air, or with the film immersed in a liquid subphase, which was pure water, with the possibility of adding hydrochloridric acid (HCl) or lithium chloride (LiCl) salts dissolved when necessary, as described in the next section. Finally, an introduction of Au wire acting as a counter electrode in the ellipsometry liquid cell allows applying a voltage between the film and the liquid phase to promote ions migration in and out of the film. The inversion of the data (ellipsometric angles Δ and Ψ) to yield the complex index of refraction n~ = n - ik and its spectral dependence was performed partly using the proprietary Wollam inversion program, for the spectroscopic measurements, and by a Matlab code developed by us implementing the calculation of the Fresnel coefficients for the single wavelength experiments. In the latter case a point-by-point inversion approach was exploited. Within the same formalism, also the expected reflection coefficients were calculated. The reflectance measurement have been taken using a reflection/ backscattering probe (Ocean Optics RP200-7) which consists of a bundle of 200 μm core optical fibers (0.22 of numerical 5830 DOI: 10.1021/la9037606
Cristofolini et al. aperture) in a “6 around 1” design: six external optical fiber are used for illumination and connected to a tungsten halogen light source (Ocean optics LS-1) while the central read fiber collects the reflected/backscattered light to the spectrometer (Ocean optics USB4000 miniature Fiber Optic Spectrometer). The probe was used at the optimal working distance of 2 mm from the measured surface. By this setup we could cover the spectral range 350-1050 nm.
3. Results and Discussion 3.1. Langmuir Monolayer. We first describe the properties of Langmuir monolayers of PANI both in its conducting and insulating forms, which are obtained by spreading the same amount of PANI either onto pure water, or onto an acidic subphase. This allows us to explore the phases of polyaniline in the most pure 2D form, in a very unconventional morphology, and free from the dead layer effects and substrate interactions which have always been a problem in analyzing the data for very thin films.15-18 Langmuir Isotherms. Pressure/area isotherms for a PANI film both on pure water subphase (PANI in the insulating emeraldine base form) and on acidic subphase (0.1 M HCl, PANI in the conducting emeraldine salt form) were routinely obtained to characterize the quality of the films. These studies were particularly important not only to yield complementary information to the ellipsometric data, but also to verify the quality of the monolayers, which as previously discussed, is an important condition for truly metallic behavior. From this point of view, it is useful to use the scaling laws analysis,19 which applies in the semidilute regime.20 In this regime the surface pressure can be expressed as Π = Γδ,21,22 where Γ is the surface concentration of molecules and δ is a scaling exponent related to the Flory exponent ν, which in turn relates the chain radius of gyration, Rg, to the number of monomers. In two dimensions, the relation between the exponents is δ = 2ν/(2ν-1). In particular one expects ν = 1 for the extended chains (selfavoiding polymer at the interface), a decreased value close to 4/7 for the condition of Θ solvent and finally ν = 1/2 for the poor solvent case, which is characterized by the presence of hard aggregates of collapsed polymeric chains. Correspondingly δ takes the values of 2, of 8-10, and of infinity. This was first verified for a polymer monolayer by Vilanove22 and has since been applied to a wide range of polymeric monolayers.23 In Figure 1, we report the pressure/area isotherm for PANI spread on pure water subphase (continuous line) in its insulating emeraldine base form, and on acidic subphase (0.1 M HCl, broken line) therefore with PANI in its conducting emeraldine salt form. The two curves are very similar, the main difference being in a shift toward smaller values of area per molecule for the emeraldine salt phase. This difference, which was consistently found in our many control experiments, might be due to some loss of substance during spreading, rather than to completely different molecular arrangement, as discussed below. From the surface isotherm data for the different films, we can extract the corresponding 2D static compression moduli;defined (15) Jerome, B.; Commandeur, J. Nature 1997, 386, 589. (16) Cristofolini, L.; Arisi, S.; Fontana, M. P. Phys. Rev. Lett. 2000, 85, 4912. (17) Cristofolini, L.; Cicuta, P.; Fontana, M. P. J. Physics: Condens. Matter 2003, 15, S1031. (18) Fakhraai, Z.; Forrest, J. A. Science 2008, 319, 600. (19) De Gennes, P. G.; Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1978. (20) Jones, R. A. L.; Richards, R. W. Polymers at surfaces and interfaces; Cambridge University Press: Cambridge, U.K., 1999. (21) Daoud, M.; Jannink, G. J. Phys. (Fr.) 1976, 37, 973. (22) Vilanove, R.; Rondelez, F. Phys. Rev. Lett. 1980, 45, 1502. (23) Cicuta, P.; Hopkinson, I. J. Chem. Phys. 2001, 114, 8659–8670.
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Figure 1. Pressure/area isotherm for PANI in both insulating (continuous line) and conductive form (broken line). Abscissae are nominal concentrations in A2 per monomer unit. Inset: Static 2D compression modulus ε, as a function of the surface concentration Γ, deduced from the same pressure area data for monolayers of PANI in the two forms, insulating (continuous line) and conductive (broken line).
as ε = Γ (∂Π/∂Γ).20 The resulting behavior is shown in the inset of Figure 1. Both insulating base and conducting salt films reach the same maximum modulus ε = 0.08(1) N/m in the semidiluted regime, thus suggesting similar mechanical properties for the two PANI films. To further investigate on the difference/similarities in the structure of the two films, in Figure 2 we show a scaling analysis of the same pressure/area isotherms: both films clearly obey the scaling law, Π = Γδ with the same coefficient δ = 11, indicating that both PANI films behave closely to the polymer in 2D Θ solvent. This, and the compression modulus behavior, indicates that the mechanical properties of the two PANI films in the semidilute regime are similar, excluding collapse of the PANI salt film, as well as other sources of heterogeneity. The main difference between the two films lies in the quantity of material that is lost during the spreading process. We are thus led to the conclusion that spreading of PANI solution on both neutral and acidic subphases results in the formation of ordered molecular layers in both cases. Optical Properties. Coming now to the optical properties of the two films, in Figure 3 we report k, the imaginary part of the complex refractive index for a Langmuir layer of PANI on a subphase made by pure water (dots in the left panel, base PANI) and on a water solution of HCl 0.1 M (dots in the right panel, PANI salt), as measured by spectroscopic ellipsometry, together with their best fits (continuous lines) to a model in which the complex dielectric constants for the insulating and conductive films, in agreement with the literature10 are respectively given by εins ðωÞ ¼ ε¥ þ Aπ -π Lπ -π ðωÞ þ Aexc Lexc ðωÞ εcond ðωÞ ¼ ε¥ þ Aπ -π Lπ -π ðωÞ þ AΠ þ LΠ þ ðωÞ þ ADrude LDrude ðωÞ in which ε¥ is the high frequency dielectric constant, Aπ-π* and Lπ-π* represent the amplitude and the Lorentzian term associated with the π-π* transition, Aexc and Lexc the amplitude and Lorentzian term associated with the excitonic transition dominating the spectrum of the insulating compound, while APþ Langmuir 2010, 26(8), 5829–5835
Figure 2. Scaling analysis of the isotherms in the semidiluted regime of PANI on pure water (insulating form, left curve in the graph), and on acidic subphase (conductive form, right curve). Note that the same scaling coefficient δ = 11 applies to both cases.
and ADrude, LPþ, and LDrude represent the amplitudes and the Lorentzians associated with the positive polaronic absorption and the Drude term present in the insulating compound. We note that we modeled the Drude term by a Lorentzian centered at zero energy, in agreement with the literature.10 The main parameters obtained by the best fitting procedure are reported in Table 1. It is interesting to note that in the present case the excitonic peak was found at 1.65 eV, i.e., red-shifted from the value found by us in the films of the very same polymer deposited onto solid substrate(2.0 eV, see below), and which in turn was in agreement with the literature.24 It seems reasonable to think that electrostatic screening due to the presence of water is responsible of such “softening” of the excitonic transition could be related to a smoother morphology of the film at the interface relatively to the deposited one. We note also that the conductive film shows a contribution from pseudo-Drude almost-free carriers which is clearly visible in the infrared part of the diagram. Consistently with the results from insulating PANI, we allowed the polaronic energy to vary, and found it to be 1.4 eV, i.e., red-shifted from the value found in the bulk (1.5 eV). We want to stress here the presence of the “metallic” Drude tail can be clearly observed in the conductive PANI, implying that, despite the morphological disorder, the electronic features of the conductive PANI are retained in this peculiar 2D form of metal. On the contrary, the π-π* transition was invariably found at the energy of 3.6 eV, in agreement with the literature.25 To validate the ellipsometric results, we also recorded reflection spectra of PANI at air/water interface as a function of the subphase pH. A volume of 50 mL of PANI 0.1 mg/mL solution in NMP/toluene (10:1 v/v) was spread on water in a Petri dish and after a suitable equilibration time needed for the solvent evaporation and the monolayer stabilization, the reflectance was measured on different subphases: on neutral pure water, and NH3 and HCl solutions at different concentrations to span different values of pH between 0 and 10. The reflectivity spectra were calibrated using the bare air-water interface reflectivity, and are shown in Figure 4, top panel, together with the expected reflectivity, (24) Epstein, A. J.; Ginder, J. M.; Zuo, F.; Bigelow, R. W.; Woo, H. S.; Tanner, D. B.; Richter, A. F.; Huang, W. S.; MacDiarmid, A. G. Synth. Met. 1987, 18, 303. (25) McCall, R. P.; Ginder, J. M.; Leng, J. M.; Ye, H. J.; Manohar, S. K.; Masters, J. G.; Asturias, G. E.; MacDiarmid, A. G.; Epstein, A. J. Phys. Rev. B 1990, 41, 5202–5212.
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Figure 3. Spectral dependence of the imaginary part of the refractive index of a Langmuir monolayer of PANI in both forms, pure (insulating, dots in the left panel) and doped (conducting, dots in the right panel). Also shown as continuous lines are the best fits described in the text. Note the presence of the characteristic Drude term in the infrared region of the conducting PANI. Table 1. Fitting Parameters for the Optical Properties of the Two PANI Films with the Model Described in the Text ε¥ insulating conducting
1.26(1) 2.44(1)
Aexc (eV2)
Eexc (eV)
ADrude (eV2)
BDrude (eV)
APþ (eV2)
EPþ (eV)
5.05(5) N.A.
1.65(5) N.A.
N.A. 2.34(5)
N.A. 0.77(5)
N.A. 1.03(5)
N.A. 1.40(5)
Eπ-π* (eV)
χ2
3.60(5) 3.60(5)
1.23 1.07
Figure 4. Top panel: comparison of the reflectivity spectra measured on the following different subphases: neutral water, water solution of NH3 with pH = 10, water solutions of HCl with pH = 0 and pH = 1.0. Note the onset of the IR reflectivity tail related to the Drude contribution. Bottom panel: comparison of the experimental reflectivity spectra with the corresponding predictions based on the complex refractive index measured by spectroscopic ellipsometry. Left: insulating PANI base, right: conducting PANI on HCl pH = 1.0 subphase. Note the qualitative agreement. 5832 DOI: 10.1021/la9037606
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calculated independently for the water/film/air system with the Jones matrix approach in the Fresnel formalism. In this calculation no adjustable parameters are involved, the film thickness being known both from the X-ray reflectivity11 and ellipsometric experiments, while the spectroscopic dependence of the refractive index is measured by spectroscopic ellipsometry. Incidentally we note here that the knowledge of the spectral dependence of the reflectivity, with an unknown scale factor given by the geometry of the particular experimental setup, in the presence of a molecular layer of unknown thickness and complex refractive index on a subphase, does not allow the unique determination of the film complex refractive index. For this reason, we choose to use the values of the refractive index determined by spectroscopic ellipsometry to compute the expected reflectivity and compare that with the experimental data. Note in the top panel of Figure 4 the maximum in the reflectivity of the insulating phase around 680 nm is related to the excitonic peak, while in the conducting phases this disappears and is replaced by the onset of the IR reflectivity tail related to the Drude contribution. In the bottom panel, we report the comparison of the experimental reflectivity spectra with the corresponding predictions based on the complex refractive index measured by spectroscopic ellipsometry. Note in particular the agreement of the spectra in the insulating phase. This is particularly noteworthy since two very different experimental techniques are involved in the two measurements. A qualitative agreement is also found for the conducting phase, since the monolayer reflection spectra shows the features of the conducting form characterized by the high NIR reflectivity, in qualitative agreement with the results obtained by spectroscopic ellipsometry. However some quantitative differences, mainly around the region of the excitonic peak, are consequence of some experimental differences in the way the two measurements are performed, perhaps leading to incomplete conversion of the PANI film to the conducting form in the reflectivity measurements. 3.2. Deposited Layers. The successful characterization of the Langmuir monolayer provides a useful benchmark to study size (e.g., thickness) effects on the conductivity properties of deposited PANI films for very small thicknesses. Another point of interest is provided by recent work26 on an electrochemically controlled device consisting of a heterojunction between an LS PANI multilayer (the active component) and a thicker solid electrolyte (Liþ doped poly(ethylene oxide) (PEO)), which is highly nonlinear and features the memory properties of a functional memristor,27,28 making it particularly interesting for applications to adaptive networks for complex information processing devices. We first characterized the substrate, which consisted of BK7 glass supporting a film of thickness 18.2(2) nm of gold deposited by evaporation. Then we studied two sets of samples, consisting of 18 Langmuir Schaeffer layers of PANI (thickness of 49(1) nm), and 44 LS layers, (thickness of 119(1) nm). From this we can estimate the thickness of a single molecular layer of PANI, which gives the result 2.7(1) nm, in good agreement with the measurements on both sets of samples and in excellent agreement with the thickness measured by synchrotron X-ray reflectivity.11 In Figure 5 we show the imaginary part of the refractive index of the sample consisting of 18 layers of PANI in air. The thick continuous line is the original sample, dominated by the strong excitonic transition, which is reported in the literature at 2 eV for
emeraldine base,24 besides the π-π* band gap transition found at 3.5-3.6 eV in agreement with the literature.25 Exposure of the molecular film to HCl vapors for 5 min induces conductivity by doping to the conducting emeraldine salt.29 The corresponding spectrum is reported in the thin continuous line, mainly dominated by the polaronic bands centered at 1.5 and 3 eV, with the contribution of pseudo-Drude almost-free carriers,9,10 which is clearly visible at wavelengths larger than 827 nm (1.5 eV), besides the π-π* transition, which is not much affected by the doping process. We also report the ellipsometric spectrum after 1 h annealing in air after HCl doping (thin dashed line in the same Figure 1): it can be noticed that the Drude tail is somewhat reduced. The same Drude tail can be restored and reach even larger values after a second exposure of the film to HCl vapors for 5 min (thick dashed line in the same Figure 5). This is in very good agreement with the behavior of the DC conductivity, as reported in ref 30, and could be related to initially inhomogeneous distribution of the doping along the film depth, which eventually becomes homogeneous after a suitable equilibration/diffusion time. On this basis, we can also confirm the presence of a pseudo-Drude term even for such thin film, in partial contrast with the findings reported in ref 10 from which it was concluded that the Drude term;indicating true conductivity;could be obtained only in PANI films of much larger thickness, e.g., 100 nm. Our result is qualitatively confirmed by the data we obtained for even thinner films, down to the single monolayer. To validate our assumption doping by HCl the PANI film was transforming it to the conducting form, we also monitored directly the electrical conductivity of the deposited PANI film before and after the doping process: the pristine film had R > 200 MΩ (resistivity F > 1 kΩ cm), which dropped after the doping to R = 16 kΩ (F = 0.08 Ω cm) . We therefore safely identify the conducting phase. We note in passing that it would be of the topmost interest to perform the same electrical characterization on the floating Langmuir monolayer, thus proving directly the
(26) Smerieri, A.; Erokhin, V.; Fontana, M. P. J. Appl. Phys. 2008, 103, 094517. (27) Chua, L. O. IEEE Trans. Circuit Theory 1971, 18, 507–519. (28) Strukov, D. B.; Snider, G. S.; Stewart, D. R.; Williams, R. S. Nature 2008, 453, 80.
(29) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277. (30) Troitsky, V. I.; Berzina, T. S.; Fontana, M. P. Mater. Sci. Eng. C 2002, 22, 239.
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Figure 5. Spectral dependence of the imaginary part of the refractive index for a sample consisting of 18 layers of PANI. The thick continuous line is the pristine film, thin continuous line after the first exposure to the doping agent HCl for 5 min, thin dashed line after subsequent annealing for 1 h in air, and finally thick dashed line after a second exposure to HCl (for 5 min).
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Figure 6. Measurement of kinetics of variation of the optical properties of a PANI film subject to doping and dedoping (see text for details) by time-resolved single wavelength TIRE measurements. The arrows mark the times at which the doping HCl solution was injected in the subphase and at which dedoping by pure water flushing was initiated. Left panels: time evolution of the ellipsometric parameters Δ (top) and Ψ (bottom). Right panel: time evolution of the deduced imaginary part of the refractive index (k). Note that at λ = 633 nm the insulating phase is characterized by its strong excitonic peak, while little absorption is found at this wavelength in the conducting phase (see previous figure, Figure 5). Therefore, the kinetic of transition to conducting phase can be followed by the decrease in magnitude of the imaginary part (k).
metallic character of this truly 2D system; however, the unavoidable conductivity of the acidic subphase makes this measurement unreliable. We could also follow in real time the process of doping by performing time-resolved single-wavelength TIRE measurements. This is made possible by the much faster single wavelength null-ellipsometer operated at the wavelength of 633 nm. We note that at this wavelength the spectrum of insulating PANI is dominated by the excitonic peak, which is absent in the conductive form. We measured a film made by 18 layers of PANI immersed in a liquid cell, which allows continuous, real time, ellipsometric measurement while the film is exposed to a liquid flow. The film was conditioned by flowing pure water for 40 min; then water was replaced for 30 min by a 0.1 M solution of HCl. Finally, the acid was washed away and pure water was flown in again. The results are shown in Figure 6 for the measured ellipsometric parameters Δ and Ψ (left panels), and the deduced imaginary part of the refractive index (right panel). The conversion from insulating to conducting form clearly emerges by the variation of the imaginary part of the refractive index (k) measured at λ = 633 nm: at this wavelength the insulating form is characterized by the strong excitonic absorption peak, while very little absorption is present in the conducting form, whose Drude tail becomes significant at much longer wavelengths, as shown in Figure 5. Therefore, we could easily measure the time constant for the conversion from insulating to conducting state, which is clearly much longer (of the order of 12 min) than that for the back conversion to insulating state (which is of the order of 1 min or less). We also note in passing that this result was obtainable thanks to the combination of enhanced sensitivity provided by the TIRE geometry and the relatively fast time scale accessible by single wavelength ellipsometry. In order to validate the results obtained by spectral ellipsometry and further investigate the relation between the conductivity and the oxidation/reduction state of PANI film, we also performed spectral reflectivity measurements on a sample consisting of 16 molecular layers of PANI deposited on glass. The PANI film, prepared in the insulating form, was subsequently converted to its conductive form by doping with immersion in a 0.1 M HCl solution. We measured the reflectivity for the film in the two states, and we compared it with the reflectivity spectra calculated knowing the film thickness and the spectral dependence of the complex refractive index determined by ellipsometry;as we had done for 5834 DOI: 10.1021/la9037606
the films on the water surface. The comparison is shown in the Supporting Information: there is good qualitative agreement, taking into account that this is not a fitting procedure and in the calculation there is no adjustable parameter; in particular we note that the reflectivity spectrum of the insulating form is determined mainly by the strong excitonic peak, while in the conducting form the Drude tail is emerging clearly. However there is not exact match between predictions and measurements, as it could be expected given that we are not performing a fit with adjustable parameters, but we are comparing different films subject to similar but not identical preparation and doping protocols. To further exemplify the power of these optical techniques in indirectly monitoring the conductivity of a PANI film, we unconventionally modulated the conductivity of a molecular layer of PANI by applying a voltage between the PANI film itself and the aqueous subphase containing an appropriate mixture of alkali metal salt such as LiCl 0.1 M together with HCl 0.1M, by an electrochemical reaction involving pushing the Li ions inside the film. At the same time the in-plane conductivity of the PANI film was monitored in a similar fashion to that reported in ref 26. In the Supporting Information we report, as an example, the evolution of the imaginary part of the optical refractive index measured by ellipsometry at 633 nm, which is determined by the electrostatic potential inducing Li ions migration outside the PANI film (when the applied voltage is positive) or inside the film (when the voltage is negative). We compare it with the evolution of the IR reflectivity (integrated in the wavelength range 960-1040 nm) in the same configuration and under the same voltage cycle. In this case, the conductive phase is characterized by an increased value of the imaginary part of the refractive index, while the insulating phase induced by the Li intercalation (Li pseudo doping29) presents a lower imaginary part of the refractive index; however the phase conversion might be not complete, and further investigations are in progress.
Conclusions We have shown, using the experimental approach of combining time-resolved ellipsometry, spectroscopic ellipsometry and multichannel optical absorption, that the complex high frequency dielectric response function of a polymeric monolayer at the air-water interface features an insulator to conducting transition, very similar to that found in thick deposited films. We could do Langmuir 2010, 26(8), 5829–5835
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this also thanks to the increased sensitivity obtained by the total internal reflection geometry for ellipsometric measurements (TIRE), which is applied here for the first time to the single wavelength case. In this way we could analyze the spectral dependence of optical properties of PANI films as a function of the doping level, and fit them to the models present in the literature. Our results suggest that in deposited PANI films the conducting behavior persists for thinner films all the way down to a few deposited monolayers, and we infer that the polymeric monolayers at the air water interface could actually be metallic. This could be due to the smoothing effect of the fluid nature of the pseudosolid PANI layer, which reduces considerably the structural and morphological defects of the solid state, as is not uncommon in polymeric molecular layers.20,31 This is confirmed by the analysis we performed on the Langmuir isotherms, both in terms of scaling law, and extracting the static 2D compression modulus, from which we could infer that insulating and conductive PANI Langmuir monolayers not only have very similar structural properties, as indicated by our independent X-ray reflectivity and GID study,11 but they also share similar mechanical properties. We then extended our study to deposited films: the optical data obtained in this case indicate that the insulating-conducting transition in PANI layers induced by doping leads to metallic (31) Cristofolini, L.; Fontana, M. P.; Berzina, T.; Konovalov, O. Phys. Rev. E 2002, 66, 041801.
Langmuir 2010, 26(8), 5829–5835
Article
films independently of the thickness. This is probably due to the higher morphological quality of LB films, and correlates well with the results obtained for the Langmuir monolayer. We could also determine the time constants for the doping and dedoping process of a PANI multilayer. Finally, we could follow by optical means, and in real time, the phase transitions of a PANI film due to the intercalation/ deintercalation of Li ions as a function of the voltage applied. Acknowledgment. We acknowledge the financial support of the Future and Emerging Technologies (FET) program within the Seventh Framework Programme for Research of the European Commission, under the FET-Open Grant Agreement BION, Number 213219. We also acknowledge Y. Gunaza for the preparation of the artwork for the TOC graphic. Furthermore, L.C. and A.N. acknowledge a British Council/CRUI grant for scientific exchange. Supporting Information Available: Figures showing spectral dependence of the reflectivity from a deposited PANI film in both insulating and conductive forms (measured and calculated) and time evolution of the imaginary part of the refractive index of a film of PANI measured at the single wavelength 633 nm by null ellipsometry, and in the NIR by reflectometry, under a voltage applied. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la9037606
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