ARTICLE pubs.acs.org/JPCC
Polarization Mediated Chemistry on Ferroelectric Polymer Surfaces Zhengzheng Zhang,† Rosette Gonzalez,†,‡ Gerson Díaz,†,‡ Luis G. Rosa,†,‡,* Ihor Ketsman,† Xin Zhang,† Pankaj Sharma,† Alexei Gruverman,†,* and Peter A. Dowben*,† †
Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska—Lincoln, Theodore Jorgensen Hall, 855 North 16th Street, Lincoln Nebraska 68588-0299, United States ‡ Department of Physics and Electronics, University of Puerto Rico—Humacao, 100 Road 908 CUH Station, Humacao, Puerto Rico 00791, and the Institute for Functional Nanomaterials, University of Puerto Rico, Facundo Bueso Building, Rio Piedras, Puerto Rico 00931 ABSTRACT: We have investigated the chemisorption of two isomers of diiodobenzene on the molecular ferroelectric copolymer poly(vinylidene fluoride) with trifluoroethylene (70:30) as a function of the polymeric ferroelectric domain orientation. We find that the 1,4-p-diiodobenzene adsorption at 150 K strongly favors a positive ferroelectric domain orientation while the 1,2-o-diiodobenzene adsorption at 150 K strongly favors a negative ferroelectric domain orientation. Both polar and nonpolar adsorbates are influenced by the substrate dipole direction. Because a nonpolar isomer is also affected by the surface dipole direction, and hence the surface chemical termination, it is clear that surface chemistry and not simply surface dipoles matter in the chemisorption process.
’ INTRODUCTION Reversible adsorption of weakly adsorbed molecules has been shown to depend on the polarization ferroelectric domain orientation of both organic1 4 and inorganic ferroelectrics.5 10 Such studies have largely investigated polar adsorbate molecules,1 10 with the tacit assumption that nonpolar molecules should be insensitive to the ferroelectric polarization domain orientation.5 7 The chemistry of the surface can play a role and it would be rare, if not unusual, for the surface chemistry of positive and negative ferroelectric domains to be identical.11,12 In this sense, nonpolar molecules would not be immune from the surface dipole direction of a ferroelectric surface. Not only would the nonpolar molecule respond to the dipole direction, as all molecules have a finite polarizability, but surface termination, stoichiometry, and defect densities can differ with different dipole (polarization) directions. This interplay between the surface chemical interactions and the electrostatic dipolar interactions can be explored by comparing adsorption of isomeric molecules that are both polar and nonpolar. The investigation of molecular isomeric effects on the adsorption of different diiodobenzene isomers on molecular zwitterion films demonstrated that the frontier orbital symmetry can play a dominant role in the adsorption process.13 For a stronger substrate adsorbate interaction, particularly reaction chemistry,8,9 it remains unclear whether it is the differences in the static surface dipole or the differences in the surface chemistry that dominates. For ferroelectrics, the reaction chemistry suggests that it is the surface dipoles that matter, but there is no clear delineation of surface chemistry and surface dipoles where the surface chemistry is photoactivated.14 26 r 2011 American Chemical Society
Here we investigated diiodobenzene adsorption/absorption on crystalline copolymers of polyvinylidene with trifluoroethylene (PVDF-TrFE), a molecular ferroelectric. As with some prior studies of adsorption on ferroelectric substrates,5 we compare a polar and nonpolar species, but by investigating two different isomers of diiodobenzene, we compare molecules with far more similar surface chemistry. If dipoles alone matter, and pyroelectricity and surface chemistry can be excluded, then it is difficult to see how an “up” or “down” poled ferroelectric should matter in an adsorption experiment, as dipole interactions hold regardless of the dipole orientation. Chemically similar adsorbate test species that have different static dipoles should show a preference for surfaces with the same or similar chemical termination (the same dipole direction preference) if surface chemistry alone matters. Thus key to any fundamental understanding is performing the experiments at constant temperature, to eliminate pyroelectric contributions to the surface charge, a complication common to ferroelectric materials. To be sure as a ferroelectric, PVDF-TrFE is also a pyroelectric so surface charge can accumulate with changes in temperature.27 At 150 K, the initial adsorption of all three isomers of diiodobenze is found to be similar on a conducting but chemically inert substrate like graphite,28 although a number of isomer specific effects have been identified for the halogenated and substituted benzenes.13,28,29 Such small simple molecules provide a clear test of preferential isomer adsorption, particularly as Received: April 15, 2011 Revised: May 31, 2011 Published: June 03, 2011 13041
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The Journal of Physical Chemistry C their intrinsic dipole depends on the isomer: the dipole moment of 1,2-diiodobenzene is the largest with an experimentally determined value of 1.87 D30 (1.856 D from density functional theory, with the PW91 exchange and correlation potential), while 1,3-diiodobenzene is intermediate with a static dipole value of 1.19 D30 (1.297 D from density functional theory), and 1,4diiodobenzene has a net dipole moment of zero. While molecular dipoles certainly have a profound influence on adsorption,1,2,31 45 here we show that dipolar interactions alone cannot explain the preferential adsorption of one isomer of diiodobenzene. Copolymers of polyvinylidene with trifluoroethylene (PVDF-TrFE), when poled up have a surface that terminates in hydrogen, and because of the copolymer replacement of 30% (CH2 CF2) with (CHF CF2) ,46,47 fluorine (15%) as well. When this ferroelectric copolymer is poled down, the surface terminates in fluorine alone.27,46 52
’ EXPERIMENTAL SECTION Ultrathin ferroelectric films of the copolymer 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE 70:30), were fabricated by Langmuir Blodgett (LB) deposition techniques on graphite substrates from the water subphase and dimethyl sulfoxide (DMSO).3,4,46 48,53 71 The films were nominally nine molecular layers thick, roughly equivalent to 30 Å thick, as determined by atomic force microscopy studies. The films were annealed in ultrahigh vacuum at 110 °C, which has proven to be an effective recipe in prior studies for preparing a clean and ordered surface3,4,46 48,53 71 and has been demonstrated to result in a surface free from impurities (including (CH2 water).71 Poly(vinylidene fluoride) [PVDF, CF2)n ] copolymers with trifluoroethylene [TrFE, (CHF CF2) ] can form highly ordered crystalline ferroelectric polymer ultrathin films as has been demonstrated by X-ray and neutron scattering,52 54,57,63,64 scanning tunneling microscopy (STM),4,46,47,53,54,61 63,66,69,70,72 low energy electron diffraction,48,63 and band mapping.4,48,63,70 Although not always evident in scanning tunneling microscopy (STM), the band structure shows a characteristic superperiodicity dominated by (CH2 CF2)2 or (CH2 CF2) (CHF CF2) “dimer” pairs4,48,63,69 in the ferroelectric phase. The copolymer P(VDF-TrFE, 70:30), despite the low overall symmetry, does show all the characteristics of high local symmetry and symmetry selection rules, with the dipoles aligned along the surface normal at lower temperatures (especially below 150 K).67,68,73 The ferroelectric behavior and polarization orientation were determined by means of piezoresponse force microscopy (PFM) measurements in ambient environment.74 Figure 1 shows the PFM hysteresis loops (phase and amplitude PFM signals as a function of applied dc bias) typical for these ferroelectric films (and indeed of ferroelectrics in general). As-grown films are weakly polarized with the dipoles generally oriented up (away from the substrate). Macroscopic poling of the P(VDF-TrFE, 70:30) films was carried out by scanning over the entire film area (of the order of square centimeters) with an electrically biased external probe (applied voltage was in the range from 800 to 800 V) held at 100 μm above the surface (noncontact poling). This noncontacting poling technique is a large scale poling of the sample similar to that achieved on a more local scale by using various scanning probe microscopy techniques on P(VDF-TrFE, 70:30) films61,62,70,75 86 and is seen to produce stable ferroelectric poling in our samples. The results of the poling process
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Figure 1. The piezoresponse force microscopy (PFM) phase and amplitude hysteresis loops measured in a 30 Å thick P(VDF-TrFE, 70:30) film, as a function of applied voltage.
were confirmed by PFM testing as in many of these prior studies of P(VDF-TrFE, 70:30) films. Combined ultraviolet photoemission (UPS) and inverse photoemission (IPES) spectra4,5,13,46,47,59,63,65,69,87 were taken in a single ultrahigh vacuum chamber to study the placement of both occupied and unoccupied molecular orbitals of the combined diiodobenzene P(VDF-TrFE, 70:30) molecular film system, as a function of diiodobenzene exposure to the samples at 150 K. The IPES spectra were obtained by using variable kinetic energy incident electrons while detecting the emitted photons at a fixed energy (9.7 eV) using a Geiger M€uller detector.4,5,13,40,46 48,63,65,69,87 The inverse photoemission spectroscopy was limited by an instrumental line width of approximately 400 meV, as described elsewhere.4,5,13,40,46 48,63,65,69,87 The angle integrated photoemission (UPS) studies were carried out using a helium lamp at hν = 21.2 eV (He I) and a Phi hemispherical electron analyzer with an angular acceptance of (10° or more, as described in detail elsewhere.4,5,13,40,46,47,59,63,65,69,87 The photoemission experiments were made with the photoelectrons collected along the surface normal, while the inverse photoemission spectra were taken with the incident electrons normal to the surface. This constraining of the electron emission (photoemission) or electron incidence (inverse photoemission) to the surface normal was done to preserve the highest point group symmetry and eliminate any wave vector component parallel with the surface. In both photoemission and inverse photoemission measurements, the binding energies are referenced with respect to the Fermi edge of gold in intimate contact with the sample surface and the photoemission (UPS, XPS) data are expressed in terms of E EF (thus making occupied state energies negative). The core level X-ray photoemission spectra were taken with a VG-Fisons X-ray source with an Al anode (hν = 1486.8 eV), and a VG 100 hemispherical analyzer, in an ultrahigh vacuum chamber also equipped with in situ facilities for noncontact poling of the ferroelectric film, as described above. Again the photoelectrons were collected along the surface normal and binding energies are referenced with respect to the Fermi edge of gold in intimate contact with the sample surface thus again making occupied state energies negative in terms of E EF. The studies were carried out at 150 K, well below the expected desorption temperature of the diiodobenzenes.13,28 Here, each isomer of diiodobenzene (Sigma-Aldrich >99% purity for 1,2diiodobenzene and 1,4-diiodobenzene; >98% purity for 1,3diiodobenzene) was admitted to the vacuum chamber through a standard leak valve and thus absorbed or adsorbed on the 13042
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Figure 2. The combined photoemission and inverse photoemission of about 3 nm thick copolymer 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE 70:30), molecular films as a function of (a) 1,2-diiodobenzene and (b) 1,4-diiodobenzene exposure. The exposures were done with the P(VDF-TrFE 70:30) molecular films maintained at 150 K, and exposure is denoted in langmuirs, where 1 langmuir = 1 10 6 Torr 3 s. The bars at the top of the panels on the right are the placement of the molecular orbitals rigidly shifted about 5 eV, approximately consistent with the work function. Binding energies are referenced with respect to the Fermi edge in terms of E EF, thus making occupied state energies negative.
P(VDF-TrFE 70:30) molecular ferroelectric films from the vapor. Exposures are denoted in langmuirs, where 1 langmuir = 1 10 6 Torr 3 s.
’ RESULTS The Adsorption of Diiodobenzene on Ferroelectric Films of the Copolymer 70% Vinylidene Fluoride with 30% Trifluoroethylene, P(VDF-TrFE 70:30). Diiodobenzene will adsorb
molecularly on thin ferroelectric films of the copolymer 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE 70:30), with the substrates held at 150 K as seen in Figure 2. As with 1,2-diiodobenzene, 1,3-diiodobenzene, and 1,4-diiodobenzene adsorption on zwitterionic p-benzoquinonemonoimine compound C6H2( 3 3 3 NHR)2( 3 3 3 O)2 where R = n-C4H9 molecular films,13 the characteristic molecular orbitals of 1,4-diiodobenzene are clearly evident in the combined photoemission and inverse photoemission, shown in Figure 2. Indeed the combined photoemission and inverse photoemission spectra for 1,4-diiodobenzene adsorption on p-benzoquinonemonoimine zwitterion molecular films13 and P(VDF-TrFE 70:30), shown here, are very similar. We believe that there is some decomposition, at least with annealing of adsorbed diiodobenzene on P(VDF-TrFE 70:30). This decomposition inferred from the strong signal of the iodine 3d core level can be observed at room temperature and above with postexposure annealing of the sample, as indicated in Figure 3 for 1,4-diidobenzene. Samples were therefore routinely replaced to avoid any contribution to the spectra from diiodobenzene fragment contamination, as a result of this dissociation that appears to be common to all of the diiodobenzenes adsorbed
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Figure 3. X-ray photoemission spectra for both the I 3d5/2 and I 3d3/2 core level features of 1,4-diiodobenzene adsorbed on 3 nm thick copolymer 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE 70:30), molecular films following exposures of 400 langmuirs at 150 K, and then followed by subsequent annealing to the temperatures indicated. The P(VDF-TrFE 70:30) molecular film has been poled up. Binding energies are referenced with respect to the Fermi edge in terms of E EF, thus making occupied state energies negative.
on the P(VDF-TrFE 70:30) substrates at 150 K. This strong chemisorption and possible decomposition are unlike with diiodobenzene absorption/adsorption on the p-benzoquinonemonoimine zwitterion molecular films at 150 K, where many adsorption and desorption cycles (>20) were required for traces of the diiodobenzene to remain at room temperature after annealing. Despite the possible fragmentation upon adsorption, the combined photoemission and inverse photoemission spectra of 1,4-diiodobenzene and the apparent HOMO LUMO gap are consistent with the molecular orbitals of the 1,4-diiodobenzene, as seen in Figure 2. There is agreement of the single molecule calculations with the combined photoemission and inverse photoemission, as seen in Figure 2. Good agreement is found between the observed (Figure 2) and calculated (top of Figure 2) energy levels of the molecular orbitals of 1,4-diiodobenzenes using a single molecule semiempirical molecular orbital calculation (PM3). There should be differences from expectations based on a single molecule and a thin film due to intermolecular interactions within the film (solid state effects) and band structure, and the semiempirical model PM3 is a simplistic semiempirical single molecule ground state calculation not corrected for matrix element effects. This type of comparison with experiments has been, however, very successful in modeling all of the isomers of diiodobenzene adsorbed/absorbed on p-benzoquinonemonoimine zwitterion molecular films,13 although, as seen here, the calculated orbital energies must be rigidly shifted in energy by about 5 eV, largely to account for the influence of work function (single molecule orbital energies are referenced to the vacuum level, while the photoemission and inverse photoemission experiments reported here are references to the Fermi level of gold in intimate contact with the sample). The initial iodine 3d5/2 core level binding energies for 1,2diiodobenzene and 1,4-diidobenzene adsorption on the ferroelectric copolymer polyvinylidene (70%) with trifluoroethylene 13043
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Figure 4. The iodine 3d5/2 core level intensities with increasing exposure of (9) 1,2-diiodobenzene, (2)1,3-diiodobenzene, and (b) 1,4-diiodobenzene to 3 nm thick copolymer 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE 70:30), molecular films maintained at 150 K. Exposures are denoted in langmuirs, were 1 langmuir = 1 10 6 Torr 3 s. The P(VDF-TrFE 70:30) molecular films have all been poled up.
(30%) are similar, with a binding energy value of 620.8 ( 0.2 eV. These values are close to the core level binding energies observed with initial 1,2-diiodobenzene (621.1 ( 0.4 eV) and 1,4-diidobenzene (621.7 ( 0.4 eV) adsorption/absorption on on p-benzoquinonemonoimine zwitterion molecular films13 and extremely similar to the binding energy of 620.7 ( 0.1 eV observed for the initial molecular adsorption of all three isomers on graphite.28 While this also suggests that the initial adsorption of diiodobenzenes on P(VDF-TrFE 70:30) at 150 K is indeed molecular, this does not exclude partial dissociation nor does this exclude strong chemisorption. What is clear is that the appearance of spectroscopic signatures of the diiodobenzene molecular orbitals requires very large exposures of 1,2-diiodobenzene, compared to 1,4-diiodobenzene, for adsorption on the P(VDF-TrFE 70:30) substrates held at 150 K. In other words, the initial sticking coefficient of 1,2-diiodobenzene on the P(VDF-TrFE 70:30) substrates held at 150 K is low compared to that for 1,4-diiodobenzene. Either 1,2-diiodobenzene does not readily adsorb on the P(VDF-TrFE 70:30) substrate or the fact that the films are naturally poled up after growth inhibits 1,2-diiodobenzene adsorption on the P(VDF-TrFE 70:30) substrates. As demonstrated below, we show that in fact the latter is true. This suggests that 1,4-diiodobenzene adsorption on P(VDFTrFE 70:30) substrates held at 150 K is more facile than 1,2diiodobenzene adsorption, as indicated by the combined photoemission and inverse photoemission spectra that is reflected in the iodine core level intensities, as summarized in Figure 4. The Influence of Ferroelectric Poling on Diiodobenzene Adsorption on Ferroelectric Films of the Copolymer 70% Vinylidene Fluoride with 30% Trifluoroethylene, P(VDFTrFE 70:30). The preferential adsorption and absorption of one particular isomer of diiodobenzene on thicker p-benzoquinonemonoimine zwitterion molecular films at 150 K is clearly evident from the iodine 3d5/2 core level signals, as well as combined photoemission and inverse photoemission.13 The strong preference for the adsorption/absorption of 1,4 diiodobenzene on P(VDFTrFE 70:30) substrates held at 150 K, as grown and not yet
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Figure 5. The iodine 3d5/2 core level intensities with increasing exposure of (a) 1,2-diiodobenzene and (b) 1,4-diiodobenzene exposures to 3 nm thick copolymer 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE 70:30) molecular films maintained at 150 K poled both up (9) and down (0). Exposures are denoted in langmuirs, were 1 langmuir = 1 10 6 Torr 3 s.
extrinsically poled (indicated by Figure 2), is still influenced by the direction of the ferroelectric domains, as seen in Figure 5. The far greater sticking coefficient evident from the I 3d5/2 intensities with 1,4-diiodobenzene than is observed with 1,2diiodobenzene exposure to P(VDF-TrFE 70:30) substrates held at 150 K, occurs with the ferroelectric poling up along the surface normal. The reverse is true when the ferroelectric polarization is directed down, as seen in Figure 5. In fact there is little difference in sticking coefficient for 1,4-diiodobenzene adsorption on P(VDF-TrFE 70:30) substrates held at 150 K and poled up and the sticking coefficient for 1,2-diiodobenzene adsorption on P(VDF-TrFE 70:30) substrates held at 150 K and poled down. The apparent differences in sticking coefficient are only apparent when diiodobenzene adsorption on the P(VDF-TrFE 70:30) substrates, held at 150 K, is compared for one poling direction, as is the case in Figure 4. Because the diiodobenzene adsorption experiments performed are isothermal, pyroelectric contributions to the adsorption process can be excluded. The surface dipole direction is still seen to have a strong influence on adsorption. Dipole interactions alone are difficult to explain as dominating the adsorption process because it is difficult to understand why up dipole and down surface dipoles would not, in principle, exert the same influence on a polar adsorbate. For a nonpolar isomer like 1,4diiodobenzene, there should be no affect of the surface dipole orientation, if the dipolar interaction is all that matters in the isothermal adsorption process. As with adsorption of the various isomers of diiodobenzene on the p-benzoquinonemonoimine zwitterion molecular films at 150 K,13 there is a strong dependence of diiodobenzene adsorption on P(VDF-TrFE 70:30) molecular films on the choice of isomer, but this isomer dependence is also strongly dependent on the direction of the ferroelectric poling. Even more important, unlike prior studies,5 7 the adsorption of the nonpolar 1,4-diiodobenzene also depends on the direction of the ferroelectric polarization. This implicates the changes in surface chemistry that occur with reversal of the ferroelectric polarization: hydrogen-terminated when poled up and fluorine terminated when poled down. The fact that the films 13044
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The Journal of Physical Chemistry C are naturally poled up after growth, even without extrinsically poling the P(VDF-TrFE 70:30) substrates, is what leads to the reduction of the 1,2-diiodobenzene adsorption on the P(VDFTrFE 70:30) substrates, as is indicated in Figure 2. Such differences in the interfacial chemistry are easy to understand for 1,2-diiodobenzene interactions with the copolymer P(VDF-TrFE 70:30). When poled down, the surface is completely terminated in fluorine, but when poled up, because of the addition of trifluorethylene, the surface is not completely hydrogen terminated. Furthermore, the spatial extent of the frontier orbitals about the iodine is far larger than that for hydrogen. So when P(VDF-TrFE 70:30) is poled up, for 1,2diiodobenzene, one iodine interacting with fluorine can inhibit the interaction of the adjacent iodine interacting with a hydrogen and this is far more prevalent and likely than with 1,4-diiodobenzene. Although this example of diiodobenzene adsorption on P(VDFTrFE 70:30) molecular films involves strong chemisorption, these results indicate that like with adsorption on inorganic ferroelectrics,88 surface chemistry cannot be a priori excluded when the ferroelectric polarization is reversed. The surface dipole direction may well matter in a significant manner but it is important to note, based on the results presented here, that the influence of surface dipolar interaction in and by itself cannot always be excluded from the accompanying changes of the surface chemical termination.
’ SUMMARY By controlling the ferroelectric polarization direction, isomer specific interfacial chemistry can be engineered. The results here show that some interfacial reaction chemistry at an organic ferroelectric interface can be turn “on” and “off” by reversal of the surface dipole orientation. Diidobenzene adsorption studies on P(VDF-TrFE 70:30) molecular films reveal that 1,2-diiodobenzene adsorption is strongly favored when the ferroelectric polarization is oriented down (toward the substrate), while 1,4-diiodobenzene adsorption is strongly favored when the polarization is up. Because these experiments are isothermal, pyroelectric contributions to this selectivity can be excluded. The implication is that while interface dipole orientation can strongly influence adsorption kinetics, interface chemistry can trump the influence of the interface dipole, as has been previously suggested.88 This selective chemistry based on phenomenon related to the ferroelectric polarization orientation should therefore be understood within models going beyond simple dipolar interactions as is suggested by recent demonstrations that the frontier orbitals matter in adsorption and chemisorption at the molecule molecule interface.13 ’ AUTHOR INFORMATION Corresponding Author
*Telephone: 402-472-9838. FAX: 402-472-6148.
’ ACKNOWLEDGMENT This research was supported by the National Science Foundation through Grants CHE-0909580 and PHY-1005071 as well as the Nebraska MRSEC (DMR-0820521). ’ REFERENCES (1) Levshin, N. L.; Yudin, S. G.; Diankina, A. P. Vestn. Mosk. Univ. Fiz. (Moscow Univ. Phys. Bull.) 1997, 52, 71–74.
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