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J. Phys. Chem. B 2007, 111, 7742-7746
Activated Water Desorption from Poly(methylvinylidene cyanide) Carolina C. Ilie,† P. A. Jacobson,† I. N. Yakovkin,*,‡ Luis G. Rosa,† Matt Poulsen,† D. Sahadeva Reddy,§ James M. Takacs,*,§ and P. A. Dowben*,† Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, Behlen Laboratory of Physics, UniVersity of Nebraska-Lincoln, Lincoln, Nebraska 68588-0111, Institute of Physics of the National Academy of Sciences of Ukraine, Prospect Nauki 46, KieV 03028, Ukraine, and Department of Chemistry, 316 Hamilton Hall, UniVersity of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304 ReceiVed: February 28, 2007; In Final Form: May 9, 2007
We have investigated water desorption from the polymer poly(methylvinylidene cyanide). The angle resolved thermal desorption spectra show large deviations from the cosn θ distribution for water desorption from poly(methylvinylidene cyanide) indicative of an activated desorption process. The Arrhenius plots obtained from Polanyi-Wigner analysis of the thermal desorption data suggest that a two-state model of desorption applies, while theory suggests that lattice strain in the polymer plays a key role in the thermal desorption of water.
Introduction Desorption of a molecule from the surface, although simple enough to observe, is a poorly understood process. A deeper understanding of the physics of molecular thermal desorption is constrained by the limited number of angular resolved thermal desorption studies. We do know that the desorption energies for molecules leaving a surface can be dependent on the desorption angle.1,2 Desorption should occur preferentially along the surface normal according to a cosn θ distribution, if ballistic phonons mediate molecular desorption. In experiments the desorption rate of a surface species can deviate from the expected cosine law and can even be enhanced off the surface normal. Off normal desorption has been observed2-5 and is likely due to steric effects. Activated desorption processes have been suggested since 19686 to explain molecular desorption rates that are often strongly peaked along the surface normal, although this model,6 like the model of different gas temperatures,7 fails to explain the experimental data.1 Extrinsic (externally) activated thermal desorption is known,3,8 in spite of the fact that the microscopic mechanisms have not been directly identified. This lightactivated (extrinsic) thermal desorption of water from poly(vinylidene fluoride-trifluoroethylene) (70:30) is also seen to lead to deviations from the expected cosn θ distribution. Here we demonstrate that there may be polymeric systems where intrinsic activated molecular desorption processes occur, possibly due to the polymer thin film lattice. The interactions of water with the copolymer poly(vinylidene fluoride-trifluoroethylene) (70:30), P(VDF-TrFE),3-5,8,9 and the related poly(methylvinylidene cyanide), PMVC,8,10 have been previously investigated as water adsorption systems. The thermal desorption of water from P(VDF-TrFE) has been observed to be enhanced with UV illumination3,8 suggesting that these systems provide a test of extrinsic activation processes in thermal desorption. Water absorption is seen to result in lattice strain (a local distortion of the polymer chains) for water absorbed in the †
Behlen Laboratory of Physics, University of Nebraska-Lincoln. Prospect Nauki 46. § Department of Chemistry, University of Nebraska-Lincoln. ‡
copolymers poly(vinylidene fluoride-trifluoroethylene)9 and the dipole ordered polymer poly(methylvinylidene cyanide).10 There should be a release of the strain energy, in the water desorption process, that could result in either local heating or an activated desorption process. Experimental Section Films of poly(methylvinylidene cyanide) (PMVC: -(CH(CH3)-C(CN)2)n-) were prepared using the LangmuirBlodgett (LB) technique from a water subphase.10-12 The LB method produces films of poly(methylvinylidene cyanide) (PMVC) with a high degree of surface crystallinity, as is evident from scanning tunneling microscopy (STM), as shown in the inset to Figure 1, and experimental band structure mappings.11 The angle resolved thermal desorption spectra were taken with a quadrupole mass spectrometer (Dycor), monitoring mass 18 amu, in an ultrahigh vacuum chamber as described in previous work.3-5,9,10 The thin films were subject to a low exposure of 15 L (1 L ) 10-6 Torr‚s) of water on PMVC. The PMVC films were 8 monolayers (ML) thick (65 Å) and were grown on graphite substrates. For the experiments undertaken, the PMVC polymer films were first annealed for 60 min at 350 K. Before each experiment, the films were annealed for 20 min at the same temperatures as the initial annealing. The films were exposed to water vapor at a substrate temperature of 160 K to avoid any complications related to surface ice formation,5,13 then the chamber was pumped for 5 min to reduce the background water vapor pressure to below 2 × 10-9 Torr. The samples were subsequently heated resistively at a rate of 0.5 deg/s. This slow heating rate was chosen in order to minimize the effect of thermal gradients in the polymer thin films. In all experiments undertaken here, the temperature of the samples was kept well below the bulk melting temperature of PMVC, which occurs at 175 °C. The angle resolved thermal desorption studies were performed at various take-off angles ranging from 0° to 50° with respect to the film surface normal. The absorption of water is seen to result in a swelling of the polymer films in XRD, and a decrease in crystallinity, as has been observed with the similar polymer poly(vinylidene fluoridetrifluoroethylene) (70:30), P(VDF-TrFE).4,5
10.1021/jp071661m CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007
Water Desorption from Poly(methylvinylidene cyanide)
Figure 1. Thermal desorption spectra of water from a 5 monolayer thick P(VDF-TrFE 70:30) film with 5 L of water exposure (a) and from an 8 monolayer thick PMVC film with 15 L of water exposure (b). The water desorbs at a higher temperature from P(VDF-TrFE) film, indicating stronger P(VDF-TrFE)-water bonding. The inset is the scanning tunneling microscopy image of nominally 3 monolayers of PMVC, adapted from ref 11.
J. Phys. Chem. B, Vol. 111, No. 27, 2007 7743
Figure 2. The integrated intensity of water desorbing from PMVC as a function of angle with respect to the surface normal, following 15 langmuirs of water exposure to a nominally 8 monolayer thick PMVC thin film. The water desorption intensity decays exponentially with increasing angle away from the surface normal, as indicated by the black curve. The curves corresponding to cosn θ, where n ∈ {1,2,3}, are also shown for comparison. PMVC polymer is shown in the inset, -(CH(CH3)-C(CN)2)n-), with C in black, N in blue, and H in white.
Thermal Desorption Spectra It has been established that water is absorbed into the thin film bulk of poly(methylvinylidene cyanide) (PMVC).10 In related studies with ferroelectric P(VDF-TrFE 70%:30%) crystalline films, absorbed bulk water was seen as a high-temperature desorption peak.3,5,8,9,13 For PMVC, desorption occurs at much lower temperatures than the desorption of absorbed bulk water from P(VDF-TrFE), for an equivalent water exposure.10 The thermal desorption spectra of water desorption from P(VDFTrFE 70:30) (shown in Figure 1a) can be compared with thermal desorption spectra from PMVC (Figure 1b). The water desorbs at a higher temperature from P(VDF-TrFE 70:30) than from PMVC, even for smaller water exposures to P(VDF-TrFE 70: 30) than for PMVC. With a small 5 langmuirs water exposure to the P(VDF-TrFE 70:30) film at 160 K, the thermal desorption of water occurs at about 375 K. With some 15 langmuirs of water exposure to PMVC at 160 K, water desorbs from nominally 8 ML PMVC films at much lower temperatures, about 292 K. Since increasing amounts of water absorption lead to higher desorption temperatures, as discussed later, the thermal desorption spectra in Figure 1 clearly indicate that absorbed water species interact more strongly with PVDF-TrFE than with PMVC,10 i.e., the absorbed water is more weakly bound to PMVC than is the case for P(VDF-TrFE).
Figure 3. Coverage dependent thermal desorption sequence from PMVC. Water is observed leaving the film after exposures of 8 L and greater. The peak center shifts with coverage from 260 to 340 K.
normal. The polymer chain displacement permits water to access bulk absorption sites. The water desorption from PMVC cannot be easily explained by simple ballistic phonon interaction, although the water molecule does interact weakly with the PMVC polymer compared with P(VDF-TrFE). Water desorption from PMVC is intrinsically activated, and though somewhat surprising, it has been seen elsewhere in thermal desorption.1,6-7 Water Absorption in PMVC
Intrinsic Activation of Water Desorption from PMVC We studied the angle resolved thermal desorption of water from poly(methylvinylidene cyanide) (PMVC), as indicated in Figure 2. The angle resolved thermal desorption spectra of water from PMVC does not follow the expected cosn θ distribution, even in the absence of UV illumination (Figure 2). Water desorption from PMVC follows an exponential decay, i.e., exp(-θ/ξ), where ξ ) 120. Deviations from the expected3 cosn θ distribution (where θ is the angle off normal and n is the critical coefficient, which can experimentally vary between 1 and 4.5,14,15 and possible with values as high as 91) in angle-resolved thermal desorption, can be indicative of an activated process. On the basis of the previous results of Rosa et al.,3 water desorption from P(VDFTrFE) is seen to be an activated process in the presence of 337 nm radiation, when the electric vector is in the plane of the surface.8 This is an extrinsic activation of water desorption. More surprising is that water desorption from PMVC needs no extrinsic activation to enhance desorption along the surface
There is additional evidence for an intrinsic activation in the thermal desorption of absorbed water from PMVC. As seen in Figure 3, at low water exposures (performed at 160 K), less than 5 L (1 L ) 1 × 10-6 Torr‚s), the thermal desorption spectra of water are featureless, indicating rapid desorption or a strong barrier to water absorption into PMVC. The evidence suggests it is the latter; the polymer chains in the thin film must distort to accommodate water absorption.10 For water exposure close to 8 L, water desorption occurs near 260 K, with an increasing desorption temperature with increasing water content in the film, as is apparent in the thermal desorption spectra. The water desorption peak from PMVC shifts to higher temperatures (from 260 to 340 K or greater) with increasing water exposure, and is attributable to the thermal desorption of an absorbed water species in PMVC.10 To investigate the possible activated desorption of absorbed water from PMVC, the thermal desorption spectra sequence was further analyzed. We used the Arrhenius-type expression for the rate constant and the Polanyi-Wigner equation. The rate
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Figure 4. Arrhenius plots derived from the thermal desorption spectra, i.e., ln(R) versus 1000/T, where R is the rate of desorption and T is the temperature. The measured activation energy E (eV) is 0.064 eV or 1.48 kcal/mol (black), 0.180 eV or 4.15 kcal/mol (red), and 0.50 eV or 11.53 kcal/mol (blue) for relative coverages of 1.5, 2.28, and 2.3, respectively.
equation for the desorption of a gas from a surface in chemical kinetics is given by the following: R(T,Θ) ) kmΘm, where Θ is the coverage, T is the temperature, m is the order of desorption, and km is the rate constant. This yields an Arrheniustype law:
( )
km ) νm(Θ) exp -
E kBT
(1a)
with νm(Θ) being the preexponential factor, E the activation energy, and kB the Boltzmann constant. Then, the rate of desorption becomes the Polanyi-Wigner equation:
( )
R(T) ) νm(Θ)[Θ(T)]m exp -
E kBT
(1b)
The Arrhenius plots, based on the integration of the thermal desorption spectra, provide some insight into the activation energy of desorption.16-20 To avoid making assumptions about the order of reaction or the rate constant, we obtained Arrhenius plots from the sequence of thermal desorption spectra as the logarithm of the water desorption rate versus the inverse of temperature (Figure 4). While the absolute water coverage is not known, relative amounts of absorbed water were estimated from the (partial) integration of water thermal desorption spectra as a function of temperature (from higher temperatures to lower temperatures). The activation energy E (eV) was then extracted from the slope, as shown in Figure 5. In Figure 5, we show the derived effective activation energy of desorption as a function of coverage, from the thermal desorption data. This analysis neglects problems associated with diffusion of water to the surface, but the features of this plot (Figure 5) do not correspond to the expected curve for a diffusion-limited system (black dotted curve). Rather the data suggest that a two-temperature model for PMVC may be applicable. Possible Origins of Activated Water Desorption from PMVC A model is needed to explain these surprising differences between the water desorption from two quite similar dipole ordered polymers. This model should be different than the Van Willigen “activated adsorption” model1,6 for hydrogen desorption. This model, based on detail balance arguments, is not in agreement with the experimental measurements done by Dabiri et al.21 Another model, proposed, but not believed as “correct” by the author (Goodman),7 assumes that the molecules have a
Figure 5. Activation energy for desorption as a function of coverage. The black dotted line represents the expected behavior for a diffusionlimited system. The red and blue dotted curves represent the fitting lines for the energy-coverage dependence.
Maxwellian-type distribution, but with different temperatures in the normal and tangential desorption directions to the surface. Willis and Fitton22 tried to explain the angular distribution of desorbing species, strongly peaked along the surface normal, as an effect of the short-range screening of the substrate electrons. The postulate is that substrate screening, for chemisorbed hydrogen on metal surfaces, leads to a weakly bound hydrogen “quasimolecule”. This latter model does not fit the kinetic energies for desorbed H2 molecules and is inapplicable to the examples presented here, as polymer thin films are good dielectrics, with no free electrons. Comsa and David23 discussed a model that has a non-cosinetype angular distribution with a non-Maxwellian gas velocity distribution but involves a superposition of two sets of desorbing particles. This cannot happen in the case where we have temperature-programmed desorption spectroscopy and the desorbed particles are obtained from a well-defined single adsorption state. In our case, if some of the absorbed water molecules are “trapped” at the surface, desorption could become a superposition of two different water states: absorbed and adsorbed water. But such a model does not address why water desorbing from PMVC10 does not exhibit a thermal desorption peak characteristic of a surface water species, as is seen from P(VDF-TrFE).3 Accordingly, the model of Comsa and David23 superficially seems excluded by the experimental data. In spite of the decades long efforts at explaining angle-resolved thermal desorption, more effort seems indicated. We believe that strain in the polymer chains may play a role in thermal desorption of water from PMVC and like polymers. As we have done previously,9 to obtain a qualitative picture of behavior of a water molecule at the PMVC polymer surface, both structure optimization and molecular dynamics (MD) simulations were undertaken using HyperChem code within a semiempirical (SE) quantum mechanical method,9 which includes parametrization of overlap integrals in the framework of the restricted Hartree-Fock (RHF) method. The semiempirical calculational approach, with the PM3 parameter set, has been recognized as a sufficiently accurate, while a very efficient method for estimating the favored structures of molecular systems, in particular polymers,9,11,13,24 by minimization of total energy, and provides results in qualitative agreement with density functional theory.13 While somewhat simplistic, this approach is also adequate for simulations modeling the evolution of molecular systems at finite temperatures by molecular
Water Desorption from Poly(methylvinylidene cyanide)
J. Phys. Chem. B, Vol. 111, No. 27, 2007 7745
Figure 7. Water molecule optimized between two chains at the hydrophobic side. The nearby CH3 groups (marked blue) turn away from the molecule thus opening the way for its desorption.
Figure 6. (a) The side view of the structure of two relaxed PMVC chains with the H2O molecule at the equilibrium position between the chains. The CH3 ligands tend to turn toward the O side of the “hands down” water molecule, thereby narrowing the desorption channel. (b) The potential barrier of 0.47 eV (11 kcal/mol) for the H2O molecule on the way from the equilibrium position between two “rigid” chains (z ) 0) to vacuum appears due to interaction with the CH3 ligands.
dynamics methods, which take into account the molecular and atomic kinetic energies. In contrast with water in the related P(VDF-TrFE),9 for the PMVC layer, the position of the water molecule between the chains is more favorable than under the layer (that is, near the hydrophilic side of the layer). In other words, the water molecule will be preferentially absorbed between the PMVC chains at the surface layer. The effective activation energy obtained from the thermal desorption spectra suggests a two-state model is appropriate. The water molecule is adsorbed and then absorbed into the sample by the straining of the polymer chains. The anisotropy of the polymer strain confers two types of strain relaxation that can lead to activated water desorption, in addition to the two sites from which the water molecule can desorb, as discussed below. This two-state model can be considered a variant of a two-temperature model discussed by Goodman.7 The water molecule absorption process, into the PMVC polymer bulk, requires some reorientation by the polymer chains, whose displacement in the proximity of the water molecule was discussed before.5,9,10 The ordering of the similar PVDF-TrFE polymer chains certainly decreases4,9 with increasing water exposure, and increased water absorption. The water molecule is admitted into the PMVC film due to the fact that the polymer chain does not remain “rigid”, but behaves in a “flexible” manner, which means that an approaching water molecule can slightly distort the PMVC chains (with a 12% increase of local spacing) so that a low potential energy configuration is created. The “hands down” orientation of the molecule located between PMVC chains near the hydrophobic side (Figure 6a) is the most favorable (that is, corresponds to the global minimum of potential energy). However, in the course of molecular dynamics modeling and optimization, other orientations such as “hands up” also were found to be favorable, and therefore the potential energy dependence on the coordinate z normal to the surface (Figure 6b) should be considered only as representative of various plots, which, nonetheless, demonstrates common features. In particular, along the desorption path, the water molecule should overcome a high (∼11 kcal/mol or ∼0.47 eV) potential barrier originating predominantly from interaction with CH3 ligands which tend to turn toward the oxygen side of the “hands down” water molecule (see Figure 6a), thereby narrowing the desorption channel. In the desorption process, should
Figure 8. Potential energy for the water molecule between PMVC chains at the film surface. Due to rotation of the CH3 ligands, the position of the water molecule, shown in Figure 8, becomes favorable (z ) 0).
the water molecule overcome the barriers to desorption, it will obtain significant kinetic energy and momentum directed along z, which may be the reason for enhancement of desorption along the surface normal. Due to flexibility of the PMVC chains, a sufficiently slow water molecule can effect reorientation of the CH3 ligands so that a new favorable position is created (Figure 7). Thus, nearby CH3 groups turn away from the molecule thus opening the way for absorption of a water molecule. It is due to this rotation of the CH3 ligands that this absorbed/adsorbed water state (which may be considered as chemisorption, because the molecule in fact stays at the surface region) becomes favorable for the water molecule. As a result of the rotation of PMVC chains, the way for the water molecule to either adsorb or desorb is open, so the potential becomes smooth, i.e., no barrier (Figure 8). Once the polymer reorientation occurs, then the desorbing molecule only has to break the adsorption bonding with the surface to leave the 0.25 eV potential well (with respect to the evaporated molecule state). The actual trajectory of a water molecule on the energy surface describing desorption will depend on how fast the chains can adapt to perturbations caused by the passing water. In spite of these dynamical contributions to the energy, not included in these plots, we may note that the plots indicate two minima, consistent with our suggestion of two states for the desorbing water molecule. In particular, unusual behavior of desorption activation energy (see Figure 5) can be attributed to changing conditions for water molecules passing between flexible PMVC chains. Thus, fast molecules will see a high potential barrier, while relatively slow molecules will adapt the chains and therefore the activation energy will be decreased. An intermediate case also may be assumed. The channel for desorption can remain open for other molecules arriving at the surface, from the bulk, and thus aiding the desorption process before back relaxation of the CH3 ligands occurs, and such molecules will desorb easily.
7746 J. Phys. Chem. B, Vol. 111, No. 27, 2007 Because of these complications, estimates of the strain energy to the activation of water desorption are difficult to estimate from simple model calculations, but the energy associated with dipole reversal has been calculated to be about 0.42 eV,25 and the strain energy should be roughly on the same scale. These values are consistent with Figure 5. Because this is a polymer system, strain could also play a role with activation of water desorption, but be manifest as a result of the polymer glass transition.26 The glass transition temperature of PMVC is not known, but the glass transition temperature of the related copolymer poly(vinylidene fluoridetrifluoroethylene) P(VDF-TrFE) is known to be -20 °C (253 K) and occurs in the amorphous phase of the copolymer,27-30 while the glass transition temperature in copolymers of vinylidenecyanide and vinyl acetate is about 170 °C (443 K).31 We anticipate that the glass transition temperature of PMVC will be at a slightly lower temperature than is observed for P(VDFTrFE) due to the slightly lower cohesive energies of the former polymer. The observed thermal desorption spectra for water from PMVC are not easily explained if the glass transition plays a dominant role in the desorption process. If the glass transition plays a dominant role in the thermal desorption of water, water desorption from PMVC should be dominated by zero-order desorption kinetics: the pre-exponential factor of eq 1b would have little or no coverage dependence (m ) 0). In other words, water diffusion to the surface would result in some continuous effective increase in the apparent desorption temperature with increasing absorbed water concentration, but this effect would be minor. A glass transition that dominates water desorption from PMVC should lead to a thermal desorption temperature for water desorption from PMCV that differs little in temperature, and an activation energy for desorption that remains nearly coverage independent,17-19 which is clearly NOT the case, as seen in Figures 3 and 5, respectively. A dominant amorphous phase would tend to make thermal desorption much more heterogeneous and water desorption from PMVC would exhibit less coverage dependence than is observed in Figure 5, apart from diffusion related processes. If the PMVC films are amorphous or deviate strongly from a crystalline or paracrystalline material, then absorption of water would not be strongly hindered at the lowest water exposure, as is observed (Figure 3 and ref 10). While the effects of the glass transition are difficult to exclude, given the desorption temperature for water, the system does have a strong tendency to crystallize, as seen in STM (Figure 1), band structure,11 and X-ray diffraction.12 Indeed disorder is expected with increasing film thickness in this10 and related dipole ordered polymer systems,3,5 yet water desorption is seen at increasingly higher temperatures with the thicker films (toward room temperature and above). The water desorption peak continues to move to temperatures increasingly farther away from the expected glass transition temperature (likely well below room temperature), where strain related to that transition would have a more profound influence. Indeed given that the glass transition is generally thought to be related to polymer disorder in the strongly dipole ordered polymers,27-31 repeated annealing should lead to irreversible effects in the thermal desorption, as defects and disorder are removed. This is not observed after the initial annealing treatments are undertaken. Basically, growth of PMVC and P(VDF-TrFE) thin films by Langmuir-Blodgett techniques is not amenable to the formation of completely amorphous long-chain polymer films.
Ilie et al. Conclusions The angle resolved thermal desorption of water from PMVC deviates from the cosn θ distribution, indicative of an activated process. The activation energy of desorption is obtained from Arrhenius plots and the features observed in energy-coverage dependence suggest that a two-state model of desorption is appropriate. The strain anisotropy plays a role in the modality for water molecules finding adsorption and absorption sites. While we cannot exclude the possibility that the glass transition may play some role in the desorption of water, this transition is expected of more amorphous polymer systems, and it is unlikely to play a dominant role in the desorption of water from this system. Acknowledgment. This research was supported by the National Science Foundation through grant no. CHE-0415421. The authors would like to acknowledge the support of as well as a number of fruitful discussions with S. Ducharme. References and Notes (1) Comsa, G.; David, R.; Rendulic, K. D. Phys. ReV. Lett. 1977, 38, 775. (2) Borca, C. N.; Welipitiya, D.; Dowben, P. A.; Boag, N. M. J. Phys. Chem. B 2000, 104, 1047-1049. (3) Rosa, L. G.; Jacobson, P. A.; Dowben, P. A. J. Phys. Chem. B 2006, 110, 7944-7950. (4) Jacobson, P. A.; Rosa, L. G.; Othon, C. M.; Kraemer, K. L.; Sorokin, A. V.; Ducharme, S.; Dowben, P. A. Appl. Phys. Lett. 2004, 84, 88. (5) Rosa, L. G.; Jacobson, P. A.; Dowben, P. A. J. Phys. Chem. B 2005, 109, 532-535. (6) van Willigen, W. Phys. Lett. 1968, 28a, 80. (7) Goodman, F. O. Surf. Sci. 1972, 30, 525. (8) Ilie, C. C.; Jacobson, P. A.; Poulsen, M.; Rosa, L. G.; SahadevaReddy, D.; Takacs, J. M.; Ducharme, S.; Dowben, P. A. MRS Symposium Proceedings, 2007, in press. (9) Rosa, L. G.; Yakovkin, I. N.; Dowben, P. A. J. Phys. Chem. B 2005, 109, 14189-14197. (10) Jacobson, P. A.; Ilie, C. C.; Yakovkin, I. N.; Poulsen, M.; Sahadeva Reddy, D.; Takacs, J. M.; Ducharme, S.; Dowben, P. A. J. Phys. Chem. B 2006, 110, 15389-15392. (11) Xiao, J.; Rosa, L. G.; Poulsen, M.; Feng, D.-Q.; Sahadeva-Reddy, D.; Takacs, J. M.; Cai, L.; Zhang, J.; Ducharme, S.; Dowben, P. A. J. Phys.: Condens. Matter 2006, 18, L155. (12) Poulsen, M.; Ducharme, S.; Sorokin, A. V.; Sahadeva-Reddy, D.; Takacs, J. M.; Wen, Y.; Kim, J.; Adenwalla, S. Ferroelectr. Lett. Sect. 2005, 32, 91-97. (13) Rosa, L. G.; Xiao, Jie; Losovyj, Ya. B.; Gao, Yi; Yakovkin, I. N.; Zeng, X. C.; Dowben, P. A. J. Am. Chem. Soc. 2005, 127, 17261. (14) Steinru¨ck, H. P.; Winkler, A.; Rendulic, K. D. J. Phys. C: Solid State Phys. 1984, 17, L311-L316. (15) Steinru¨ck, H. P.; Luger, M.; Winkler, A.; Rendulic, K. D. Phys. ReV. B 1985, 32, 5032. (16) Kautto, E.; Kuhalainen, J.; Manninen, M. Phys. Scr. 1997, 55, 628. (17) Grunze, M.; Strasser, G.; Golze, M.; Hirschwald, W. J. Vac. Sci. Technol. A 1987, 5 (4), 527-533. Golze, M.; Grunze, M.; Hirschwald, W. Vacuum 1981, 31, 697. (18) King, D. A. Surf. Sci. 1977, 64, 43. (19) King, D. A. Surf. Sci. 1975, 47, 384. (20) Varma, S.; Dowben, P. A. J. Vac. Sci. Technol. A 1990, 8, 2605. (21) Dabiri, A. E.; Lee, T. J.; Stickney, R. E. Surf. Sci. 1971, 26, 522. (22) Willis, R. F.; Fitton, B. Astrophys. Space Sci. 1975, 34, 57. (23) Comsa, G.; David, R. Chem. Phys. Lett. 1977, 38, 512-5. (24) Feng, D.-Q.; Caruso, A. N.; Schulz, D. L.; Losovyj, Ya. B.; Dowben, P. A. J. Phys. Chem. B 2005, 109, 16382. (25) Cai, L.; Wang, X.; Darici, Y.; Dowben, P. A.; Zhang, J. J. Chem. Phys. 2007, 126, 124908. (26) Lewis, P. A.; de Reggi, A. S. In The Applications of Ferroelectric Polymers; Wang, T. T., Herbert, J. M., Glass, A. M., Eds.; Blackie and Sons: Glasgow, 1988; Chapter 7, pp 163-167. (27) Bharti, V.; Zhang, Q. M. Phys. ReV. B 2001, 63, 184103. (28) Furukawa, T.; Tajitsu, Y. Y.; Zhang, X. Ferroelectrics 1992, 135, 401. (29) Lovinger, J.; Furukawa, T.; Davis, G. T.; Broadhurst, M. G. Polymer 1983, 24, 1225. (30) Cheng, Z-Y.; Zhang, Q. M.; Bateman, F. B. J. Appl. Phys. 2002, 392, 6749. (31) Wang, T. T.; Takase, Y. J. Appl. Phys. 1987, 62, 3466.
