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HREELS study of vapor-deposited polyaniline on Ag(110)
Surface Science 420 (1999) L115–L121
Surface Science Letters
HREELS study of vapor-deposited polyaniline on Ag(110) K.K. Lee a, J.M. Vohs a, *, N.J. DiNardo b a Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA b Drexel University, Department of Physics and Atmospheric Science, Philadelphia, PA 19104, USA Received 7 August 1998; accepted for publication 20 October 1998
Abstract Ultra-thin films of polyaniline on Ag(110) were characterized using high-resolution electron energy loss spectroscopy (HREELS ) in the vibrational and electronic loss regimes. The films were grown in situ by chemical vapor deposition using an emeraldine source. Initially, short polyaniline oligomers adsorb on the surface followed by polymerization to form longer polyaniline chains. The loss spectra in the far-infrared region indicate that the films become highly conducting upon protonation. In addition, the role of the substrate in the initial stages of polymer film growth is demonstrated by comparison with polyaniline on Cu and Au surfaces. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapor deposition; Electron energy loss spectroscopy (EELS ); Growth; Metal–polymer interfaces; Silver; Single crystal surfaces
1. Introduction Polyaniline is an important member of the class of electrically-conducting conjugated polymers that have great potential for applications ranging from flat-panel displays to chemical sensors [1–8]. In the majority of these applications, polyaniline is used in the form of a thin film. Therefore, it is important to understand relationships between film growth, structure, and electronic properties as well as the role of the substrate. We have recently ˚) investigated the growth of ultra-thin (<100 A polyaniline films on Cu(110) and on polycrystalline Au surfaces by analyzing vibrational and electronic excitation spectra acquired by highresolution electron energy loss spectroscopy * Corresponding author. Fax: +1 215 573 2093; e-mail: [email protected].
(HREELS) [9–13]. The films were fabricated by vapor deposition of polyaniline oligomers produced by the evaporation of emeraldine, which is the half-oxidized form of polyaniline. Emeraldine has the following chemical formula:
In the initial stages of growth, our previous studies indicated a distinct bonding interaction of the aniline oligomers with the Cu surfaces and weaker bonding to the Au surfaces. As the film thickness was increased, detailed analysis of the vibrational spectra suggested an increase in chain length, or polymerization of the adsorbed oligomers. The appearance of a far-infrared plasmon loss in the electronic loss spectra indicated that
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subsequent ex situ doping of the thin films with HCl formed highly conducting polyaniline. In this work, we study the growth of ultra-thin polyaniline films on an Ag(110) single crystal surface with HREELS in the vibrational and electronic loss regimes. HREELS vibrational and electronic spectra show that film growth proceeds via adsorption of aniline oligomers followed by polymerization to form longer polymer chains. In addition, loss spectra in the far-infrared region indicate that the films become highly conducting upon ex situ protonation. This work provides a comparison with spectra of polyaniline films grown on Cu(110) and on polycrystalline Au to investigate the role of the substrate surface on growth.
2. Experimental The experiments were performed in an ultrahigh vacuum chamber equipped with an LK Technologies Model 3000 HREEL spectrometer. The experimental system has been described in detail previously [11]. The Ag(110) single crystal surface was cleaned in situ by repeated cycles of Ar+ sputtering followed by annealing to 900 K. This process was continued until the surface exhibited a sharp (1×1) low energy electron diffraction (LEED) pattern. Growth of the polyaniline thin films was achieved by exposing the Ag(110) surface to polyaniline vapor produced by evaporation of emeraldine. The polymer source was contained in an auxiliary chamber connected to the main UHV analysis chamber by a gate valve and consisted of a resistively heated quartz cell filled with emeraldine base powder. A thermocouple was attached to the cell to allow for temperature measurement. Heating the quartz cell to 650 K produced a polyaniline vapor pressure of 1×10−6 torr. After the source was thoroughly outgassed, the substrate was exposed to the polyaniline vapor by opening the gate valve separating the auxiliary chamber from the main chamber. Detailed deposition procedures were described in our previous studies [11,12]. Exposures to polyaniline vapor are reported in Langmuirs (1 L=10−6 torr s). The vibrational spectra presented in this study were collected in a specular
scattering geometry with a 4 eV incident electron energy with the beam directed 60° from the surface normal. The full-width at half-maximum of the elastic peak in the HREEL spectra was typically 3 meV. Electronic loss spectra were recorded with a 15 eV incident electron energy (~65 meV resolution) with the same scattering geometry.
3. Results and discussion Fig. 1 displays HREEL vibrational spectra of the polymer films as a function of polyaniline vapor exposure at a sample temperature of 300 K. Spectrum (a) in Fig. 1 was obtained after a 5 L polyaniline vapor exposure, and contains vibrational losses characteristic of the adsorbed organic polymer film. Since the film is expected to contain polyaniline oligomers of various lengths and degrees of oxidation, it is difficult to assign all the
Fig. 1. HREEL vibrational spectra of Ag(110) exposed to (a) 5 L, (b) 20 L, and (c) 40 L of polyaniline vapor at 300 K. The inset shows an expanded view of the 2500–3600 cm−1 region. Spectra were obtained with electron beam energy of 4 eV and FWHM of the elastic peak ~24 cm−1.
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peaks unambiguously. In general, however, the peaks between 600 and 1200 cm−1 are due to C–H bending modes, those between 1200 and 1400 cm−1 are due to C–N stretching modes, and those between 1400 and 1600 cm−1 are due to ring stretching modes. The C–H stretching modes occur between 2900 and 3100 cm−1 and the N–H stretching modes occur between 3100 and 3500 cm−1. The large peak at 2130 cm−1 can be assigned to the symmetric stretching mode of adsorbed CO, which is contained in the chamber background gas during exposure. The presence of this peak indicates that the 5 L polyaniline exposure does not produce complete coverage of the Ag(110) surface, and this provides a means to estimate film thickness. Several of the more prominent peaks in the spectrum provide insight into the chemical structure of the polyaniline oligomer layer at this low coverage. In particular, the large peak at 751 cm−1 is at an energy characteristic of the C– H out-of-plane bending mode of a monosubstituted benzene ring [14,15]. Note that this peak is ~2.5 times as intense as the peak at 824 cm−1, which is at an energy characteristic of a C–H outof-plane bending mode of a paradisubstituted benzene ring. The relative intensities of the monosubstituted and paradisubstituted C–H out-of-plane bending modes agree with infrared spectra of aniline oligomers containing three rings or fewer [16–19]. This indicates that the film contains a large number of end groups and is therefore composed primarily of relatively short polyaniline oligomers. This result is consistent with our previous studies of the vapor deposition of polyaniline on Cu and Au substrates [11,12]. Spectra (b) and (c) of Fig. 1 are for thicker films obtained following polyaniline exposures of 20 and 40 L, respectively. These spectra provide insight into the evolution of the chemical structure of the polymer layer during the initial stages of film growth. The intensities of the C–N stretches and the ring stretches increase significantly with coverage; however, the most noticeable spectral changes for the thicker film occur in the C–H bending and C–H stretching regions as discussed below. The stretching mode of adsorbed CO is not present in the spectra for the higher polyaniline
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exposures. This indicates that the Ag(110) surface is entirely covered by the polymer layer for a 20 L exposure. Increasing the polyaniline dose from 5 to 20 L produces a dramatic change in the relative intensities of the C–H out-of-plane bending modes centered at 751 and 824 cm−1. The ratio of the intensities of the C–H bending modes of monosubstituted rings (751 cm−1) to those of paradisubstituted rings (824 cm−1) decreased from ~2.5 for the 5 L film to ~1.0 and ~0.9 for the 20 and 40 L films, respectively. Assuming that the spectrum for the submonolayer 5 L exposure is characteristic of the polyaniline oligomers produced by the emeraldine source, the increase in the relative intensity of the paradisubstituted species with film thickness can be attributed to polymerization of the adsorbed oligomers to form longer chains. This conclusion is consistent with previous IR spectroscopy studies of bulk polyaniline [14], and phenylterminated aniline tetramers [16 ] and with our previous HREELS studies of polyaniline film growth on Cu and Au substrates [11,12]. Changes in the intensities of the C–H bending modes were accompanied by changes in the C–H stretching region. For the 5 L film, the C–H stretching region contains a broad, asymmetric peak centered at 3040 cm−1, while two peaks of nearly equal intensity can be resolved at 2910 and 3055 cm−1 in the spectrum of the 20 L film. These peaks can be assigned to symmetric and asymmetric C–H stretches, respectively. The C–H region of the spectrum of the 40 L film also contains symmetric and asymmetric stretching modes centered at 2910 and 3055 cm−1; however, the symmetric stretch is nearly twice as intense as the asymmetric stretch. The different intensities of C– H stretching modes further indicate changes in the chemical structure of the polymer layer with thickness. This observation is in agreement with the conclusion that adsorbed polyaniline oligomers react to form longer polymer chains during chemical vapor deposition [11,13]. It is interesting to compare the vibrational spectrum of vapor-deposited polyaniline on Ag with those obtained for similar films on Cu and Au substrates. Fig. 2 shows vibrational spectra for polyaniline films on Ag(110) (40 L exposure),
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phologies giving rise to the observed differences in the vibrational spectra. In addition to the vibrational spectra, electronic loss spectra were measured. Fig. 3 shows electronic spectra obtained following polyaniline exposures of 5, 20, and 40 L. For the 5 L dose (a), the spectrum exhibits a large symmetric peak centered at 3.7 eV. This peak corresponds to the plasmon mode of the Ag(110) surface [20]. A small broad feature is also apparent at 6.8 eV; this can be attributed to the adsorbed polymer layer [21]. Increasing the polyaniline exposure to 20 and 40 L (b and c, respectively) results in an increase in the intensity of the 6.8 eV peak and a significant broadening of the high energy side of the plasmon peak at 3.7 eV. This broadening suggests the appearance of an additional loss between 3.8 and 4.5 eV. As reported previously, the electronic loss spectra of undoped, vapor-deposited polyaniline films on Au and Cu substrates exhibit peaks at 2.5, 4.2, and 6.8 eV [11]. Similarly, Litzelmann Fig. 2. Vibrational loss spectra of Ag(110), Cu(110) and Au substrates exposed to ~50 L of polyaniline vapor at 300 K.
Cu(110) and Au (~50 L exposures) [11]. Several differences in the spectra indicate the influence of the substrate on the structure of these ultra-thin films. The vibrational spectra of films grown on Au and Ag are most similar. Differences between Cu and Ag or Au are observed in the C–H bending region (between 600 and 1200 cm−1) and in the C–H stretching region (between 2800 and 3200 cm−1). In particular, the polyaniline film on Cu exhibits an intense C–H bending mode centered at 940 cm−1, which is nearly absent in the films grown on Ag and Au. The relative intensities of the C–H symmetric and asymmetric modes are also significantly different on Cu compared with those on Au and Ag. The unique features for polyaniline on Cu are likely due to this substrate being more reactive than Au and Ag. Van der Waals interactions may be the primary mode of adhesion for the polymer films on Ag and Au. Differences in the mode of adhesion may produce variations in chain conformations and/or film mor-
Fig. 3. Electronic loss spectra of emeraldine film formed by exposing the Ag(110) substrate to (a) 5 L, (b) 20 L, (c) 40 L of polyaniline vapor at 300 K, and (d) HCl-doped emeraldine film at 300 K. Spectra were obtained with electron beam energy of 15 eV and FWHM of the elastic peak ~65 meV.
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et al. reported that the electronic spectra of electrochemically-prepared emeraldine films exhibit features at 2.2, 4.0 and 7.0 eV [21]. The peak near 4.0 eV arises from a p–p* transition across the polyaniline band-gap [21]. Although a distinct peak cannot be resolved at ~4 eV in the present study because of the intense Ag plasmon mode, the change in the peak shape with higher coverage strongly suggests the existence of an analogous feature in this region. The peak at 6.8 eV can be attributed to excitation of localized p-electrons in the aromatic rings [21]. A peak near 2.2 eV, which was previously assigned to an n–p* transition from non-bonding lone-pair states on the imine nitrogens to the conduction band [22], was not observed in the present study. The position of this peak is expected to shift from 2.5 to 2.0 eV with increasing conjugation length [23]. Due to the intense sloping background below 2 eV, a small loss in this spectral region may not be detectable. It should be noted, however, that this loss was resolved in HREEL spectra of vapor-deposited polyaniline films on Cu and Au substrates even with a similar sloping background [11]. Therefore, it is possible that the absence of this loss for vapor-deposited polyaniline on the Ag(110) surface is due to a different degree of oxidation compared with emeraldine. Indeed, this spectrum is similar to that reported by Cao et al., for highly-reduced aniline oligomers [17]. This result is also consistent with previous studies of the vapor deposition of polyaniline, which have indicated that this procedure results in highly reduced material [24,25]. Electronic loss spectra of the 40 L polyaniline thin film were also recorded after ex situ doping with HCl to assess the local electrical conductivity of the film. In this set of experiments, the polyaniline film was grown in the vacuum chamber and then removed from vacuum and exposed to HCl vapor for 1 h. The protonated film was then re-introduced into the vacuum chamber for further analysis. We note that in previous studies [11], exposure of the film to air (without doping) produces minimal spectral changes. The low energy spectral regions of the as-deposited polyaniline film at 300 K and the protonated film at 115 and 300 K are displayed in Fig. 4. The spectrum of the 40 L as-deposited film (a) is re-scaled from Fig. 1
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Fig. 4. HREEL spectra of (a) as-deposited ultra-thin polyaniline film on Ag(110) at 300 K, and HCl-doped ultra-thin polyaniline film on Ag(110) at (b) 115 K and (c) 300 K.
so the vibrational modes are not seen. The data in Fig. 4 shows that protonation of the polyaniline film creates rather intense tails on both the high and low energy sides of the elastic peak. These features are similar to HREEL spectra of doped semiconductors and can be attributed to a continuum of plasmon losses (i.e. excitations of conduction band electrons) [26 ]. The presence of these plasmon features indicates protonation of the film produces free charge carriers. This is an important result since it demonstrates that vapor deposition of relatively short aniline oligomers can produce conducting polyaniline films. Discrete vibrational modes are not resolved in the spectra of the doped film due to two effects. First, the dopant-induced free charge carriers screen the oscillating dipole moments of the vibrational modes. Second, coupling between the plasmon and vibrational modes results in a significant broadening of the vibrational losses. It is significant that a discrete plasmon loss is observed in the spectrum of the doped film cen-
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tered at 165 cm−1 in addition to the loss continuum. Discrete plasmon losses between 50 and 300 cm−1 are characteristic of highly conducting forms of polyaniline, such as polyaniline doped with camphor sulphonic acid (CSA) processed in m-cresol [27]. The conducting form of polyaniline is generally thought to consist of both crystalline and disordered regions. It has been suggested that conduction electrons that are delocalized between more than one crystalline region give rise to the discrete plasmon modes in the far-IR [28]. Since these plasmons represent the response of only a small fraction of the conduction band electrons, their frequencies are much lower than that expected for a Drude metal [28]. The presence of a discrete plasmon loss at 165 cm−1 in Fig. 4 of the HCl-doped polyaniline film therefore suggests that the film is highly conducting. This behavior was also observed in our previous studies after HCl doping of polyaniline films on Cu and Au substrates [11]. The temperature dependence of the plasmon losses provides insight into the electronic properties of the doped polyaniline film. As shown in Fig. 4, cooling from 300 to 115 K results in a significant decrease in the intensity of the plasmon loss continuum on the low and high energy side of the elastic peak. This indicates a decrease in the number of free charge carriers with decreasing temperature and is consistent with a ‘semiconductor-like’ behavior for the electrons that give rise to the plasmon continuum. In contrast, the intensity of the discrete plasmon loss at 165 cm−1 is nearly invariant with temperature, indicating a ‘metal-like’ behavior. These results are consistent with a highly-conducting HCl-doped ultra-thin polyaniline film on Cu(110) [11]. Electronic loss spectra of a 40 L vapor-deposited polyaniline film both before and after protonation are compared in Fig. 3 (spectra c and d). Protonation of the polyaniline film results in the appearance of two new peaks at 1.7 and 3.5 eV. In addition, the relative intensity of the Ag plasmon mode at 3.7 eV decreases significantly after protonation, making it easier to resolve the feature near 4.2 eV. This spectrum is similar to that reported by Litzelmann et al. for electrochemicallydeposited emeraldine thin films which had been
treated in buffer solutions with pH<5 [21,22]. The two peaks that emerge upon protonation near 1.7 and 3.5 eV have previously been attributed to polaron states, which are thought to be responsible for conduction in polyaniline [28].
4. Conclusions In further demonstrating that high-quality polyaniline films can be produced by the chemical vapor deposition of aniline oligomers, this study shows that the substrate surface has a distinct role in the initial stages of formation of an ultra-thin polymer film. Aniline oligomers appear to form strong covalent bonds with Cu surfaces but not with Ag or Au surfaces. HREELS vibrational and electronic spectra demonstrate that film growth proceeds via adsorption of short aniline oligomers followed by polymerization to form longer polyaniline chains. Although relatively low molecular weight material is produced by this method, the loss spectra in the far-infrared region indicate that the films become highly conducting upon protonation.
Acknowledgements We thank Professor Yen Wei of Drexel University for providing the emeraldine base powder. We also thank Professor Mark Barteau of University of Delaware for providing the metal substrate Ag(110) single crystal. The HREEL spectrometer used in this study was acquired using funds provided by the National Science Foundation through Grant DMR-9303459. We also acknowledge NSF support via the Materials Research Laboratory (MRSEC ) Program through Grant DMR-9632598 and through Grant DMR-9313047 (N.J.D.).
References [1] H. Stubb, E. Punkka, J. Paloheimo, Mater. Sci. Eng. 10 (1993) 85. [2] R.K. Yuan, Z.P. Gu, H. Yuan, Synth. Met. 69 (1995) 233.
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K.K. Lee et al. / Surface Science 420 (1999) L115–L121 [3] Y. Yang, E. Westerweele, C. Zhang, P. Smith, J. Heeger, J. Appl. Phys. 77 (1995) 694. [4] H.L. Wang, F. Huang, A.G. MacDiarmid, Y.Z. Wang, D.D. Gebler, A.J. Epstein, Synth. Met. 80 (1996) 97. [5] M.K. Ram, R. Gowri, B.D. Malhotra, J. Appl. Polym. Sci. 63 (1997) 141. [6 ] P.N. Bartlett, P.B.M. Archer, S.K. Ling-Chung, Sensors and Actuators 19 (1989) 125. [7] P.N. Bartlett, S.K. Ling-Chung, Sensors and Actuators 20 (1989) 287. [8] G.E. Collins, L.J. Buckley, Synth. Met. 78 (1996) 93. [9] R.V. Plank, N.J. DiNardo, J.M. Vohs, Surf. Sci. 340 (1995) L971. [10] R.V. Plank, Y. Wei, N.J. DiNardo, J.M. Vohs, Chem. Phys. Lett. 263 (1996) 33. [11] R.V. Plank, N.J. DiNardo, J.M. Vohs, Synth. Met. 89 (1997) 1. [12] R.V. Plank, N.J. DiNardo, J.M. Vohs, Phys. Rev. B 55 (1997) 55. [13] R.V. Plank, N.J. DiNardo, J.M. Vohs, J. Vac. Sci. Technol. A15 (1997) 538. [14] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, Macromolecules 21 (1988) 1297. [15] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met. 24 (1988) 231. [16 ] L.W. Shacklette, J.F. Wolf, S. Gould, R.H. Baughman, J. Chem. Phys. 88 (1988) 3955.
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[17] Y. Cao, S. Li, Z. Xue, D. Guo, Synth. Met. 16 (1986) 305. [18] S. Quillard, G. Louarn, J.P. Buisson, S. Lefrant, Synth. Met. 49/50 (1992) 525. [19] S. Quillard, G. Louarn, S. Lefrant, A.D. MacDiarmid, Phys. Rev. B 50 (1994) 50. [20] M. Rocca, F. Moresco, U. Valbusa, Phys. Rev. B 45 (1992) 45. [21] A. Litzelmann, B. Scheerer, J. Fink, K. Meerholz, J. Heinze, N.S. Sariciftci, H. Kuzmany, Synth. Met. 29 (1989) E313. [22] Y.H. Kim, C. Foster, J. Chiang, A.J. Heeger, Synth. Met. 29 (1989) E285. [23] I.A. Misurkin, T.S. Zhuravleva, V.M. Geskin, V. Gulbinas, S. Pakalnis, V. Butvilos, Phys. Rev. B 49 (1994) 49. [24] T.R. Dillingham, D.M. Cornelison, E. Bullock, J. Vac. Sci. Technol. A12 (1994) 2436. [25] K. Uvdal, M. Logdlund, P. Dannetun, L. Bertilsson, S. Stafstrom, W.R. Salaneck, Synth. Met. 29 (1989) E451. [26 ] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. [27] M. Reghu, Y. Cao, D. Moses, A.J. Heeger, Phys. Rev. B 47 (1993) 47. [28] A.J. Epstein, J. Joo, R.S. Kohlman, G. Du, A.G. MacDiarmid, E.J. Oh, Y. Min, J. Tsukamoto, H. Kaneko, J.P. Pouget, Synth. Met. 65 (1994) 149.
Surface Science Letters
HREELS study of vapor-deposited polyaniline on Ag(110) K.K. Lee a, J.M. Vohs a, *, N.J. DiNardo b a Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA b Drexel University, Department of Physics and Atmospheric Science, Philadelphia, PA 19104, USA Received 7 August 1998; accepted for publication 20 October 1998
Abstract Ultra-thin films of polyaniline on Ag(110) were characterized using high-resolution electron energy loss spectroscopy (HREELS ) in the vibrational and electronic loss regimes. The films were grown in situ by chemical vapor deposition using an emeraldine source. Initially, short polyaniline oligomers adsorb on the surface followed by polymerization to form longer polyaniline chains. The loss spectra in the far-infrared region indicate that the films become highly conducting upon protonation. In addition, the role of the substrate in the initial stages of polymer film growth is demonstrated by comparison with polyaniline on Cu and Au surfaces. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapor deposition; Electron energy loss spectroscopy (EELS ); Growth; Metal–polymer interfaces; Silver; Single crystal surfaces
1. Introduction Polyaniline is an important member of the class of electrically-conducting conjugated polymers that have great potential for applications ranging from flat-panel displays to chemical sensors [1–8]. In the majority of these applications, polyaniline is used in the form of a thin film. Therefore, it is important to understand relationships between film growth, structure, and electronic properties as well as the role of the substrate. We have recently ˚) investigated the growth of ultra-thin (<100 A polyaniline films on Cu(110) and on polycrystalline Au surfaces by analyzing vibrational and electronic excitation spectra acquired by highresolution electron energy loss spectroscopy * Corresponding author. Fax: +1 215 573 2093; e-mail: [email protected].
(HREELS) [9–13]. The films were fabricated by vapor deposition of polyaniline oligomers produced by the evaporation of emeraldine, which is the half-oxidized form of polyaniline. Emeraldine has the following chemical formula:
In the initial stages of growth, our previous studies indicated a distinct bonding interaction of the aniline oligomers with the Cu surfaces and weaker bonding to the Au surfaces. As the film thickness was increased, detailed analysis of the vibrational spectra suggested an increase in chain length, or polymerization of the adsorbed oligomers. The appearance of a far-infrared plasmon loss in the electronic loss spectra indicated that
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subsequent ex situ doping of the thin films with HCl formed highly conducting polyaniline. In this work, we study the growth of ultra-thin polyaniline films on an Ag(110) single crystal surface with HREELS in the vibrational and electronic loss regimes. HREELS vibrational and electronic spectra show that film growth proceeds via adsorption of aniline oligomers followed by polymerization to form longer polymer chains. In addition, loss spectra in the far-infrared region indicate that the films become highly conducting upon ex situ protonation. This work provides a comparison with spectra of polyaniline films grown on Cu(110) and on polycrystalline Au to investigate the role of the substrate surface on growth.
2. Experimental The experiments were performed in an ultrahigh vacuum chamber equipped with an LK Technologies Model 3000 HREEL spectrometer. The experimental system has been described in detail previously [11]. The Ag(110) single crystal surface was cleaned in situ by repeated cycles of Ar+ sputtering followed by annealing to 900 K. This process was continued until the surface exhibited a sharp (1×1) low energy electron diffraction (LEED) pattern. Growth of the polyaniline thin films was achieved by exposing the Ag(110) surface to polyaniline vapor produced by evaporation of emeraldine. The polymer source was contained in an auxiliary chamber connected to the main UHV analysis chamber by a gate valve and consisted of a resistively heated quartz cell filled with emeraldine base powder. A thermocouple was attached to the cell to allow for temperature measurement. Heating the quartz cell to 650 K produced a polyaniline vapor pressure of 1×10−6 torr. After the source was thoroughly outgassed, the substrate was exposed to the polyaniline vapor by opening the gate valve separating the auxiliary chamber from the main chamber. Detailed deposition procedures were described in our previous studies [11,12]. Exposures to polyaniline vapor are reported in Langmuirs (1 L=10−6 torr s). The vibrational spectra presented in this study were collected in a specular
scattering geometry with a 4 eV incident electron energy with the beam directed 60° from the surface normal. The full-width at half-maximum of the elastic peak in the HREEL spectra was typically 3 meV. Electronic loss spectra were recorded with a 15 eV incident electron energy (~65 meV resolution) with the same scattering geometry.
3. Results and discussion Fig. 1 displays HREEL vibrational spectra of the polymer films as a function of polyaniline vapor exposure at a sample temperature of 300 K. Spectrum (a) in Fig. 1 was obtained after a 5 L polyaniline vapor exposure, and contains vibrational losses characteristic of the adsorbed organic polymer film. Since the film is expected to contain polyaniline oligomers of various lengths and degrees of oxidation, it is difficult to assign all the
Fig. 1. HREEL vibrational spectra of Ag(110) exposed to (a) 5 L, (b) 20 L, and (c) 40 L of polyaniline vapor at 300 K. The inset shows an expanded view of the 2500–3600 cm−1 region. Spectra were obtained with electron beam energy of 4 eV and FWHM of the elastic peak ~24 cm−1.
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peaks unambiguously. In general, however, the peaks between 600 and 1200 cm−1 are due to C–H bending modes, those between 1200 and 1400 cm−1 are due to C–N stretching modes, and those between 1400 and 1600 cm−1 are due to ring stretching modes. The C–H stretching modes occur between 2900 and 3100 cm−1 and the N–H stretching modes occur between 3100 and 3500 cm−1. The large peak at 2130 cm−1 can be assigned to the symmetric stretching mode of adsorbed CO, which is contained in the chamber background gas during exposure. The presence of this peak indicates that the 5 L polyaniline exposure does not produce complete coverage of the Ag(110) surface, and this provides a means to estimate film thickness. Several of the more prominent peaks in the spectrum provide insight into the chemical structure of the polyaniline oligomer layer at this low coverage. In particular, the large peak at 751 cm−1 is at an energy characteristic of the C– H out-of-plane bending mode of a monosubstituted benzene ring [14,15]. Note that this peak is ~2.5 times as intense as the peak at 824 cm−1, which is at an energy characteristic of a C–H outof-plane bending mode of a paradisubstituted benzene ring. The relative intensities of the monosubstituted and paradisubstituted C–H out-of-plane bending modes agree with infrared spectra of aniline oligomers containing three rings or fewer [16–19]. This indicates that the film contains a large number of end groups and is therefore composed primarily of relatively short polyaniline oligomers. This result is consistent with our previous studies of the vapor deposition of polyaniline on Cu and Au substrates [11,12]. Spectra (b) and (c) of Fig. 1 are for thicker films obtained following polyaniline exposures of 20 and 40 L, respectively. These spectra provide insight into the evolution of the chemical structure of the polymer layer during the initial stages of film growth. The intensities of the C–N stretches and the ring stretches increase significantly with coverage; however, the most noticeable spectral changes for the thicker film occur in the C–H bending and C–H stretching regions as discussed below. The stretching mode of adsorbed CO is not present in the spectra for the higher polyaniline
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exposures. This indicates that the Ag(110) surface is entirely covered by the polymer layer for a 20 L exposure. Increasing the polyaniline dose from 5 to 20 L produces a dramatic change in the relative intensities of the C–H out-of-plane bending modes centered at 751 and 824 cm−1. The ratio of the intensities of the C–H bending modes of monosubstituted rings (751 cm−1) to those of paradisubstituted rings (824 cm−1) decreased from ~2.5 for the 5 L film to ~1.0 and ~0.9 for the 20 and 40 L films, respectively. Assuming that the spectrum for the submonolayer 5 L exposure is characteristic of the polyaniline oligomers produced by the emeraldine source, the increase in the relative intensity of the paradisubstituted species with film thickness can be attributed to polymerization of the adsorbed oligomers to form longer chains. This conclusion is consistent with previous IR spectroscopy studies of bulk polyaniline [14], and phenylterminated aniline tetramers [16 ] and with our previous HREELS studies of polyaniline film growth on Cu and Au substrates [11,12]. Changes in the intensities of the C–H bending modes were accompanied by changes in the C–H stretching region. For the 5 L film, the C–H stretching region contains a broad, asymmetric peak centered at 3040 cm−1, while two peaks of nearly equal intensity can be resolved at 2910 and 3055 cm−1 in the spectrum of the 20 L film. These peaks can be assigned to symmetric and asymmetric C–H stretches, respectively. The C–H region of the spectrum of the 40 L film also contains symmetric and asymmetric stretching modes centered at 2910 and 3055 cm−1; however, the symmetric stretch is nearly twice as intense as the asymmetric stretch. The different intensities of C– H stretching modes further indicate changes in the chemical structure of the polymer layer with thickness. This observation is in agreement with the conclusion that adsorbed polyaniline oligomers react to form longer polymer chains during chemical vapor deposition [11,13]. It is interesting to compare the vibrational spectrum of vapor-deposited polyaniline on Ag with those obtained for similar films on Cu and Au substrates. Fig. 2 shows vibrational spectra for polyaniline films on Ag(110) (40 L exposure),
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phologies giving rise to the observed differences in the vibrational spectra. In addition to the vibrational spectra, electronic loss spectra were measured. Fig. 3 shows electronic spectra obtained following polyaniline exposures of 5, 20, and 40 L. For the 5 L dose (a), the spectrum exhibits a large symmetric peak centered at 3.7 eV. This peak corresponds to the plasmon mode of the Ag(110) surface [20]. A small broad feature is also apparent at 6.8 eV; this can be attributed to the adsorbed polymer layer [21]. Increasing the polyaniline exposure to 20 and 40 L (b and c, respectively) results in an increase in the intensity of the 6.8 eV peak and a significant broadening of the high energy side of the plasmon peak at 3.7 eV. This broadening suggests the appearance of an additional loss between 3.8 and 4.5 eV. As reported previously, the electronic loss spectra of undoped, vapor-deposited polyaniline films on Au and Cu substrates exhibit peaks at 2.5, 4.2, and 6.8 eV [11]. Similarly, Litzelmann Fig. 2. Vibrational loss spectra of Ag(110), Cu(110) and Au substrates exposed to ~50 L of polyaniline vapor at 300 K.
Cu(110) and Au (~50 L exposures) [11]. Several differences in the spectra indicate the influence of the substrate on the structure of these ultra-thin films. The vibrational spectra of films grown on Au and Ag are most similar. Differences between Cu and Ag or Au are observed in the C–H bending region (between 600 and 1200 cm−1) and in the C–H stretching region (between 2800 and 3200 cm−1). In particular, the polyaniline film on Cu exhibits an intense C–H bending mode centered at 940 cm−1, which is nearly absent in the films grown on Ag and Au. The relative intensities of the C–H symmetric and asymmetric modes are also significantly different on Cu compared with those on Au and Ag. The unique features for polyaniline on Cu are likely due to this substrate being more reactive than Au and Ag. Van der Waals interactions may be the primary mode of adhesion for the polymer films on Ag and Au. Differences in the mode of adhesion may produce variations in chain conformations and/or film mor-
Fig. 3. Electronic loss spectra of emeraldine film formed by exposing the Ag(110) substrate to (a) 5 L, (b) 20 L, (c) 40 L of polyaniline vapor at 300 K, and (d) HCl-doped emeraldine film at 300 K. Spectra were obtained with electron beam energy of 15 eV and FWHM of the elastic peak ~65 meV.
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et al. reported that the electronic spectra of electrochemically-prepared emeraldine films exhibit features at 2.2, 4.0 and 7.0 eV [21]. The peak near 4.0 eV arises from a p–p* transition across the polyaniline band-gap [21]. Although a distinct peak cannot be resolved at ~4 eV in the present study because of the intense Ag plasmon mode, the change in the peak shape with higher coverage strongly suggests the existence of an analogous feature in this region. The peak at 6.8 eV can be attributed to excitation of localized p-electrons in the aromatic rings [21]. A peak near 2.2 eV, which was previously assigned to an n–p* transition from non-bonding lone-pair states on the imine nitrogens to the conduction band [22], was not observed in the present study. The position of this peak is expected to shift from 2.5 to 2.0 eV with increasing conjugation length [23]. Due to the intense sloping background below 2 eV, a small loss in this spectral region may not be detectable. It should be noted, however, that this loss was resolved in HREEL spectra of vapor-deposited polyaniline films on Cu and Au substrates even with a similar sloping background [11]. Therefore, it is possible that the absence of this loss for vapor-deposited polyaniline on the Ag(110) surface is due to a different degree of oxidation compared with emeraldine. Indeed, this spectrum is similar to that reported by Cao et al., for highly-reduced aniline oligomers [17]. This result is also consistent with previous studies of the vapor deposition of polyaniline, which have indicated that this procedure results in highly reduced material [24,25]. Electronic loss spectra of the 40 L polyaniline thin film were also recorded after ex situ doping with HCl to assess the local electrical conductivity of the film. In this set of experiments, the polyaniline film was grown in the vacuum chamber and then removed from vacuum and exposed to HCl vapor for 1 h. The protonated film was then re-introduced into the vacuum chamber for further analysis. We note that in previous studies [11], exposure of the film to air (without doping) produces minimal spectral changes. The low energy spectral regions of the as-deposited polyaniline film at 300 K and the protonated film at 115 and 300 K are displayed in Fig. 4. The spectrum of the 40 L as-deposited film (a) is re-scaled from Fig. 1
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Fig. 4. HREEL spectra of (a) as-deposited ultra-thin polyaniline film on Ag(110) at 300 K, and HCl-doped ultra-thin polyaniline film on Ag(110) at (b) 115 K and (c) 300 K.
so the vibrational modes are not seen. The data in Fig. 4 shows that protonation of the polyaniline film creates rather intense tails on both the high and low energy sides of the elastic peak. These features are similar to HREEL spectra of doped semiconductors and can be attributed to a continuum of plasmon losses (i.e. excitations of conduction band electrons) [26 ]. The presence of these plasmon features indicates protonation of the film produces free charge carriers. This is an important result since it demonstrates that vapor deposition of relatively short aniline oligomers can produce conducting polyaniline films. Discrete vibrational modes are not resolved in the spectra of the doped film due to two effects. First, the dopant-induced free charge carriers screen the oscillating dipole moments of the vibrational modes. Second, coupling between the plasmon and vibrational modes results in a significant broadening of the vibrational losses. It is significant that a discrete plasmon loss is observed in the spectrum of the doped film cen-
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tered at 165 cm−1 in addition to the loss continuum. Discrete plasmon losses between 50 and 300 cm−1 are characteristic of highly conducting forms of polyaniline, such as polyaniline doped with camphor sulphonic acid (CSA) processed in m-cresol [27]. The conducting form of polyaniline is generally thought to consist of both crystalline and disordered regions. It has been suggested that conduction electrons that are delocalized between more than one crystalline region give rise to the discrete plasmon modes in the far-IR [28]. Since these plasmons represent the response of only a small fraction of the conduction band electrons, their frequencies are much lower than that expected for a Drude metal [28]. The presence of a discrete plasmon loss at 165 cm−1 in Fig. 4 of the HCl-doped polyaniline film therefore suggests that the film is highly conducting. This behavior was also observed in our previous studies after HCl doping of polyaniline films on Cu and Au substrates [11]. The temperature dependence of the plasmon losses provides insight into the electronic properties of the doped polyaniline film. As shown in Fig. 4, cooling from 300 to 115 K results in a significant decrease in the intensity of the plasmon loss continuum on the low and high energy side of the elastic peak. This indicates a decrease in the number of free charge carriers with decreasing temperature and is consistent with a ‘semiconductor-like’ behavior for the electrons that give rise to the plasmon continuum. In contrast, the intensity of the discrete plasmon loss at 165 cm−1 is nearly invariant with temperature, indicating a ‘metal-like’ behavior. These results are consistent with a highly-conducting HCl-doped ultra-thin polyaniline film on Cu(110) [11]. Electronic loss spectra of a 40 L vapor-deposited polyaniline film both before and after protonation are compared in Fig. 3 (spectra c and d). Protonation of the polyaniline film results in the appearance of two new peaks at 1.7 and 3.5 eV. In addition, the relative intensity of the Ag plasmon mode at 3.7 eV decreases significantly after protonation, making it easier to resolve the feature near 4.2 eV. This spectrum is similar to that reported by Litzelmann et al. for electrochemicallydeposited emeraldine thin films which had been
treated in buffer solutions with pH<5 [21,22]. The two peaks that emerge upon protonation near 1.7 and 3.5 eV have previously been attributed to polaron states, which are thought to be responsible for conduction in polyaniline [28].
4. Conclusions In further demonstrating that high-quality polyaniline films can be produced by the chemical vapor deposition of aniline oligomers, this study shows that the substrate surface has a distinct role in the initial stages of formation of an ultra-thin polymer film. Aniline oligomers appear to form strong covalent bonds with Cu surfaces but not with Ag or Au surfaces. HREELS vibrational and electronic spectra demonstrate that film growth proceeds via adsorption of short aniline oligomers followed by polymerization to form longer polyaniline chains. Although relatively low molecular weight material is produced by this method, the loss spectra in the far-infrared region indicate that the films become highly conducting upon protonation.
Acknowledgements We thank Professor Yen Wei of Drexel University for providing the emeraldine base powder. We also thank Professor Mark Barteau of University of Delaware for providing the metal substrate Ag(110) single crystal. The HREEL spectrometer used in this study was acquired using funds provided by the National Science Foundation through Grant DMR-9303459. We also acknowledge NSF support via the Materials Research Laboratory (MRSEC ) Program through Grant DMR-9632598 and through Grant DMR-9313047 (N.J.D.).
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