J. Phys. Chem. C 2007, 111, 13951-13956
13951
Effect of Ferroelectric Poling on the Adsorption of 2-Propanol on LiNbO3(0001) Y. Yun,† L. Kampschulte,‡ M. Li,† D. Liao,† and E. I. Altman*† Department of Chemical Engineering, Yale UniVersity, New HaVen, Connecticut 06520 and Department for Earth and EnVironmental Sciences, Ludwig-Maximilians-UniVersity, Munich, Germany ReceiVed: May 31, 2007; In Final Form: July 12, 2007
The effect of ferroelectric poling on the surface chemistry of ferroelectric materials was studied through the adsorption and temperature-programmed desorption (TPD) of 2-propanol on the positively and negatively poled LiNbO3(0001) surfaces. Although only molecular adsorption and desorption were observed on both surfaces, the desorption peak temperature of 2-propanol from the positive surface was over 100 K higher than that from the negative surface, indicating that the polar molecule adsorbs more strongly on the positive surface. Furthermore, the 2-propanol desorption peak temperature depended unusually strongly on the heating rate, yielding apparent desorption pre-exponentials on the order of 10 s-1 for the positive surface and 10-1 s-1 for the negative surface. These unusually low numbers could be associated with the pyroelectric behavior of LiNbO3 creating a temperature-dependent surface dipole; interaction of the polar molecule with this surface dipole then causes the desorption activation energy to also depend on temperature. Fitting the data to a simple linear model between the desorption activation energy and temperature suggests that the desorption activation energy of 2-propanol from LiNbO3 increases by about 0.25 kJ/mol-K as the temperature increases. Taking this effect into account, the data indicate an 11 kJ/mol higher desorption activation energy from the positive surface. The results also revealed a sensitivity of the adsorption/desorption to the morphology of the opposite face of the crystal.
1. Introduction Although ferroelectric materials are important in numerous areas of broad technological significance, their intrinsic surface physical and chemical properties have not been systematically studied.1-4 As a result, their surface structure, adsorption properties, and charge compensation mechanisms under vacuum and ambient conditions remain the subject of speculation. The ordered arrangement of dipoles in the bulk that creates the macroscopic electric field also leads to polar surfaces. Crystals oriented to expose polar surfaces generally exhibit massive surface reconstructions in vacuum that eliminate the thicknessdependent electric field that would otherwise cause the electrostatic surface energy to diverge.5 Under ambient conditions, adsorption is considered to screen the thickness-dependent electric field.6 Because opposite charges must be compensated on the positively and negatively poled surfaces, adsorption and surface reactivity are expected to depend markedly on the polarization direction. The ability to switch the polarization direction by application of an external electric field distinguishes ferroelectric materials from other materials cut to expose polar surfaces. This opens up the intriguing possibility of switching and patterning surface chemical properties. In prior work, Giocondi and Rohrer showed that the photoelectrochemical activity of BaTiO3 is sensitive to the poling direction: a property they exploited to pattern Ag lines produced by photochemical reduction of Ag+ in solution.7 Earlier work by Inoue and co-workers suggested that the activation energy for CO oxidation over Pd supported on LiNbO3 depended on the poling direction of the ferroelectric support.8 Recently, it has been suggested that adsorption of † ‡
Yale University. Ludwig-Maximilians-University.
different molecules may stabilize opposite poling directions in ferroelectric thin films, raising the possibility of creating ferroelectric chemical sensors.6 While these studies have raised interesting possibilities, the potential to manipulate catalytic reactions and create chemical sensors relies on intrinsic differences in the adsorption and reaction properties between pristine positively and negatively poled ferroelectric surfaces that have not been studied. Therefore, we have been characterizing the geometric, electronic, and chemical properties of clean ferroelectric surfaces in ultrahigh vacuum (UHV).9 In this paper it will be shown that 2-propanol adsorbs much more strongly on positively poled LiNbO3(0001) than on the negatively poled surface. Lithium niobate crystallizes in a hexagonal structure that closely resembles the corundum structure, except with one-half the cations being Li and the other half Nb.10-12 Below the Curie temperature of ∼1470 K10,13 the Li and Nb cations are displaced in opposite directions away from the centrosymmetric positions along [0001], creating dipoles that organize into ferroelectric domains. We previously showed that both the positively and negatively poled (0001) surfaces are (1 × 1) and appear predominantly covered by O atoms.9 These results were explained by an adatom-vacancy model in which oxygen anion vacancies and ad-anions form on the opposing surfaces. It was estimated that roughly one oxygen adatom (vacancy) per unit cell would be required to cancel the thickness-dependent electric field; because LiNbO3(0001) surface unit cells contain three O atoms, the surfaces remain (1 × 1). Isopropyl alcohol was adopted as the probe molecule to investigate the surface chemistry of LiNbO3. Adsorption and decomposition of alcohols has long been used as a test reaction for oxide catalysts,14-16 where acidic surfaces lead to dehydration into alkenes17 while basic surfaces favor decomposition into
10.1021/jp074214f CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2007
13952 J. Phys. Chem. C, Vol. 111, No. 37, 2007 CO and CO2.16 In between, alcohols dehydrogenate into ketones and aldehydes.18 Bulk termination of LiNbO3(0001) would expose Nb and Li cations on opposite surfaces. Because Li oxides are basic while Nb oxides tend to act as Lewis acids,19 it was anticipated that reversing the poling direction could cause a change in the desorption products. In this paper, it will be shown using temperature-programmed desorption (TPD) that 2-propanol desorbs molecularly from LiNbO3(0001) independent of the poling direction. Molecular desorption from the positive surface, however, peaked at temperatures more than 100 K higher than the negative surface, indicating much stronger adsorption on the positive surface. In addition, the ferroelectric properties of the bulk led to unusual adsorption-desorption phenomena. These included abnormally low apparent desorption pre-exponentials and an odd sensitivity of the surface properties to the morphology of the back side of the crystal. It will be shown that the former can be explained by the pyroelectric effect, i.e., the change in ferroelectric polarization with temperature, creating a temperature-dependent heat of adsorption. The implications of the results on the feasibility of creating ferroelectric chemical sensors will be discussed. 2. Experimental Section The experiments were performed using an ultrahigh vacuum (UHV) chamber equipped with a differentially pumped microwave electron cyclotron resonance plasma source (Tectra, Gen II Plasma Source), an X-ray source, a cylindrical mirror analyzer, and a differentially pumped quadrupole mass spectrometer.20 The base pressure of the system was less than 2 × 10-10 Torr. The samples were mounted on transferable sample holders that incorporated thermocouple contacts and a heater. The samples were pressed against a resistively heated Ta foil by alumina sample retaining clips. A thin sapphire wafer separated the heater from a Cu plate that could be cooled by contact to a Cu block on the sample docking station, which in turn was connected to a liquid nitrogen reservoir through Cu braids. This arrangement allowed samples to be cooled to below about 190 K. The temperature was measured using a chromel-alumel thermocouple attached to the front face of the sample using a ceramic adhesive (ultra-temp 516, Aremco Products, Inc). Nearly stoichiometric LiNbO3 (0001) samples were obtained from Almaz Optics. Samples were 8 × 6 × 0.5 mm plates polished on either one side or both sides. The samples were oriented to within (0.25° of (0001). The +z direction was determined by sandwiching the crystals between Si electrodes and measuring the sign of the voltage deflection when the samples were mildly heated with a heat gun (ferroelectric materials develop a voltage when their temperature is changed due to the pyroelectric effect).10 In keeping with convention, the positive face was assigned to that which developed a negative potential when heated. Samples were smoothed by annealing in flowing air at 1073 K for 5 h before use as described by Saito et al.21 After insertion into the vacuum system, the primary contaminant visible on the sample with XPS was carbon. Carbon was burned off by annealing to ∼575 K in the oxygen plasma for 1 h. As described previously, this preparation procedure yields surfaces with sharp (1 × 1) diffraction patterns.9 2-Propanol (J. T. Baker, A. C.) was placed in a glass vial attached to the leak valve. High vapor pressure contaminants and dissolved gases were removed by several freeze-pumpthaw cycles, and the vapor purity was then checked with mass
Yun et al. spectrometry. Exposure of the sample surface to the adsorbate was conducted by positioning the LiNbO3 sample directly in front of a capillary array doser attached to a stainless steel tube and a leak valve and then opening the leak valve, to introduce vapor into the chamber. On the basis of a comparison of the times required to reach certain CO coverages on Pd(100) with the sample positioned in front of and rotated away from the doser, it was determined that the effective pressure in front of the doser was 80 times higher than the background pressure. Thermal desorption was performed line-of-sight with the sample positioned ∼1 mm from the front of a cone fitted around the ionizer of the mass spectrometer. The mass spectrometer was differentially pumped by a 30 L/s ion pump and nonevaporable getter pump rated at 70 L/s. The mass spectrometer is capable of monitoring up to 8 mass-to-charge ratios during a TPD run. The mass-to-charge ratios were selected based on preliminary temperature-programmed mass spectra (TPMS) in which the complete mass spectra were repeatedly, rapidly collected while the sample temperature was ramped. 3. Results Comparison of TPMS data with mass spectra obtained when 2-propanol was leaked into the chamber revealed that desorption produced only the cracking fragments expected for 2-propanol in proportions consistent with molecular desorption. During TPD experiments the following mass to charge ratios were monitored: m/q ) 45 (CH3HCOH+), 15 (CH3+), 18 (H2O+), 39 (C3H3+), 41 (C3H5+), 43 (H3CCdO+), and 58 (H3CCdOCH3+). The ratios of these species were similar for positive and negative surfaces and did not vary significantly with dose, indicating that desorption was solely molecular within the detection limits of the experimental arrangement. All the TPD curves that follow in this paper show desorption at m/q ) 45, the major cracking fragment for 2-propanol. Figure 1 shows a comparison of 2-propanol desorption curves for positively and negatively poled LiNbO3 (0001) surfaces for samples polished on one side. The 2-propanol was dosed with the samples at ∼190 K; the heating rate was 1 K/s. As shown in Figure 1a, 2-propanol desorption from the positive surface was dominated by a peak at 480 K; the weaker feature at ∼240 K was attributed to desorption from the Ta wires on the sample holder. Other than an increase in the intensity of the 480 K peak, the curves did not significantly change as the coverage was increased. The constant peak temperature implies first-order desorption of molecularly adsorbed 2-propanol with only weak interactions between adsorbed molecules. This conclusion is supported by our recent observation that H atoms on positively and negatively poled LiNbO3(0001) surfaces readily reduce the surfaces below 450 K;9 thus, dissociative adsorption would produce water prior to the molecular desorption. The TPD results for the negative surface are shown in Figure 1b. In this case, the dominant feature in the curves is a desorption peak centered at 370 K; again, the peak at ∼240 K was attributed to desorption from the Ta wires. The peak at 370 K did not shift when the coverage was increased, implying that this peak also reflects a first-order coverage dependence on the desorption rate. In addition to the 110 K lower desorption temperature than the positive surface, the peak for the negative surface is also somewhat broader. Surprisingly, another peak at ∼500 K began to appear at high exposures (>10 000 L). Although this peak is in the same region as the desorption peak for the positive surface, it cannot be ascribed to patches of positive domains on the surface; since Figure 1 suggests similar
Adsorption of 2-Propanol on LiNbO3(0001)
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Figure 1. TPD traces for 2-propanol on positively (a) and negatively (b) poled LiNbO3 (0001) surfaces. The samples were polished on one side. Data were collected using a heating rate of 1 K/s while monitoring the signal at m/q ) 45 (CH3HCOH+) for 2-propanol.
Figure 2. Adsorption uptake of 2-propanol on the positively and negatively poled LiNbO3 (0001) surfaces at 190 K plotted as a function of exposure. The coverages were calculated by integrating the TPD curves in Figure 1 and normalized to the saturation coverage on the positive surface.
sticking coefficients on the positive and negative surfaces, if this were the case the peaks would at worst be expected to fill simultaneously. The relatively high temperature suggests adsorption at defect sites; however, it is unclear why the sticking coefficient at these sites would be so low or why adsorbed 2-propanol could not diffuse to these energetically favored sites. The uptake curves in Figure 2 were obtained by integrating the TPD curves in Figure 1, excluding the low-temperature desorption due to the heating leads. For these curves, 1 ML was defined as the saturation coverage on the positive surface. The curves indicate that at 190 K relatively high exposures were required to saturate the surfaces, close to 10 000 L for the positive surface. The data also indicate that the sticking coefficients on both surfaces were nearly constant until the coverage reached roughly 60% of saturation, suggesting that adsorption proceeds through a precursor mechanism. Although it was not possible to quantify the exact saturation coverage, an estimate of the initial sticking coefficient can be obtained
by assuming that one molecule adsorbs per surface unit cell at saturation. This suggests initial sticking coefficients of 1.6 × 10-4 and 1.0 × 10-4 for the positive and negative surfaces, respectively. The most obvious difference between the uptake curves for the positive and negative surfaces is the lower saturation coverage observed on the negative surface. This difference may be due to the broad tail of the main desorption peak for the negative surface extending down to the 190 K adsorption temperature. The data in Figure 1 show that the main 2-propanol desorption peak is 110 K higher on the positive surface than on the negative surface, indicating much stronger adsorption on the positive surface. To quantify the difference in adsorption strength, TPD curves were recorded as a function of heating rate over the range 0.25-4 K/s. Figure 3 shows a comparison of the heating rate dependence of the TPD curves for the positive and negative surfaces. The data show that as the heating rate was increased from 0.25 to 4 K/s, the desorption peaks shifted to higher temperatures: by 170 K for the positive surface and 219 K for the negative surface. These are unusually large shifts. For typical values of 1013 s-1 for the pre-exponential and 125 kJ/mol for the desorption activation energy, peak shifts of less than 50 K are expected. Nonetheless, the large shifts are not due to experimental artifacts. With the exception of the 1 K/s curve for the positive surface, the leading edges of the curves lie on top of each other as they should. The peak desorption temperatures were independent of coverage, indicating that desorption was first order. For firstorder desorption the kinetic parameters can be obtained from the slope and intercept of a plot of the log of β/TP2 versus 1/TP, where β is the heating rate and Tp the desorption peak temperature.22 In this case, the slope corresponds to -E/R and the intercept to νR/E, where R is the molar gas constant, ν the desorption pre-exponential, and E the desorption activation energy, which is minus the heat of adsorption for nonactivated adsorption. The peak temperatures observed in Figure 3 are plotted in this way in Figure 4. The steeper slope for the positive surface again indicates a higher desorption activation energy on the positive surface. The slopes and intercepts, however, suggest unphysically low values for the pre-exponentials and desorption activation energies. For example, the data suggest pre-exponentials on the order of 10 s-1 for the positive surface
13954 J. Phys. Chem. C, Vol. 111, No. 37, 2007
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Figure 3. Series of 2-propanol TPD curves measured at varying heating rates on positively (a) and negatively (b) poled LiNbO3 (0001) surfaces at exposures of 9000 and 8000 L, respectively. Since the peak temperatures were independent of coverage, the different exposures do not influence the results. The samples were polished on one side. Heating was stopped at ∼650 K to prevent severe desorption of Li2O from the sample surface region.9
interaction with the surface dipole, then a temperature-dependent heat of adsorption, and thus desorption activation energy, would be expected. Using a simple linear relationship between the desorption activation energy and temperature as a first approximation, i.e., E ) E0 + R(T - T0), where T0 is a reference temperature, E0 the desorption activation energy at T0, and R a proportionality constant, the following is produced for the firstorder desorption rate constant
κ ) νe-E/RT ) νe-[E0+R(T-T0)]/RT ) νe-R/Re-(E0-RT0)/RT ) νeffe-Eeff/RT
Figure 4. Log-scale plot of β/Tp2 vs Tp for desorption of 2-propanol on positively (a) and negatively (b) poled LiNbO3 (0001) surfaces. Data were obtained from the heating rates (β) and corresponding desorption peak temperatures (Tp) in Figure 3. The steeper slope for the positive surface is indicative of a higher desorption activation energy.
and 10-1 s-1 for the negative surface, or 13 orders of magnitude lower than the typical value for small molecules. An inherent assumption in the above analysis is that the desorption activation energy does not change with temperature. Lithium niobate, however, is intrinsically pyroelectric, meaning its spontaneous polarization decreases with increasing temperature. As described in the Introduction, the ferroelectric effect creates polar surfaces that would have divergent electrostatic surface energies. The observation of stable surfaces requires that the divergent electrostatic surface energy must have been screened as the surfaces were prepared. If the surfaces were perfectly screened prior to adsorption, the pyroelectric effect would create an excess electric field at the surface as the temperature is changed,23 provided perfect screening is not instantaneous. If adsorption of 2-propanol is driven by its
where νeff and Eeff are the effective desorption pre-exponential and activation energy observed in TPD experiments. A desorption activation energy that changes by only 0.25 kJ/mol-K is sufficient to create an effective desorption pre-exponential 13 orders of magnitude lower than the intrinsic pre-exponential. Assuming a typical intrinsic pre-exponential of 1013 s-1,22 the data in Figure 4 then suggest that R is between 0.25 and 0.27 kJ/mol-K for 2-propanol on LiNbO3 (0001). Setting T0 to the starting temperature of the TPD ramp, 200 K, then suggests E0 is 73 kJ/mol on the positive surface and 62 kJ/mol on the negative surface. The data in Figures 1-4 were all collected using samples polished only on one side. Curiously, smaller differences were seen between the positive and negative surfaces of samples polished on both sides. Figure 5a shows a TPD curve for 2-propanol desorption from the positive surface of a sample polished on both sides. The curve was obtained using a heating rate of 1 K/s and showed a dominant peak at 310 K and a higher temperature shoulder between 400 and 500 K; the curve for the negative surface was similar except for a weaker hightemperature tail. Surfaces of samples polished on both sides did not show any significant differences from samples polished on one side in XPS, ISS, and diffraction experiments, and results similar to those presented in Figures 1-5 were obtained for multiple samples. To confirm that the difference between samples polished on one side and both sides was not an anomaly, the back side of the sample used to collect the data in Figure 5a was roughened using a bead blaster operated with 100 µm
Adsorption of 2-Propanol on LiNbO3(0001)
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Figure 5. Typical TPD traces for 2-propanol desorption from the (a) positively poled LiNbO3 (0001) surface of a sample polished on both sides and (b) positively poled surface of a crystal with the back side roughed by bead blasting. The curves were collected using a heating rate of 1 K/s while monitoring the signal at m/q ) 45 (CH3HCOH+) for 2-propanol.
SiO2 particles. The TPD experiments were then repeated with the results for a 1500 L dose shown in Figure 5b. The figure shows two distinct desorption peaks, at 330 and 450 K. More importantly, comparing the results with those in Figures 5a and 1a shows that roughening the back side made the sample appear more similar to the sample that was only ever polished on one side, indicating that this unusual effect where the condition of the back side of the sample affects the behavior of the opposite surface is not due to an experimental artifact. 4. Discussion The results reveal several unusual phenomena: a desorption peak temperature that can be changed by over 100 K by switching the polarization direction, a desorption pre-exponential many orders of magnitude lower than typically observed, and a sensitivity of the adsorption/desorption to the morphology of the opposite face of the crystal. All of these phenomena can be linked to the ferroelectric and pyroelectric properties of LiNbO3. The ferroelectric effect creates polar surfaces with a divergent electrostatic surface energy that is typically screened by accumulation of negative charges on the positive surface and positive charges on the negative surface. In UHV, the charges may come from reconstructions involving anion vacancies (or excess cations) on the negative surface and excess anions (or cation vacancies) on the positive surface. Alternatively, adding electrons to the positive surface and removing them from the negative surface can also screen the thickness-dependent electric field.24 It should be noted that the screening charges could accumulate on the surfaces during surface preparation and subsequent characterization, for example, by oxygen anion adsorption from the plasma onto the positive surface. Where the charges come from, however, does not change the number of charges required to screen the ferroelectric field, making it difficult to spectroscopically determine their source. Since opposite charges accumulate on the positive and negative surfaces, it is not surprising that adsorption of a polar molecule such as 2-propanol is sensitive to the poling direction. Interestingly, when the thickness-dependent electric field is screened, negative charges accumulate on the positive surface and positive charges on the negative surface; thus, the higher
desorption temperatures for the positive surface suggest that 2-propanol interacts more strongly with negative surface charges. Further, since the pyroelectric effect causes the bulk polarization to decrease with increasing temperature, if the surfaces were initially screened at the adsorption temperature, the compensating charge densities would be in excess of those required to screen the bulk field as the temperature is increased. As a result, large surface dipoles in the opposite direction of the poling direction can build up as the temperature is ramped during the TPD runs. It is therefore understandable how the molecules might become more strongly bound to the surface as the temperature is increased. The effect of temperature on the surface dipole moment can be estimated from the physical properties of LiNbO3.9 The spontaneous polarization of stoichiometric LiNbO3 is reported as 80 ( 5 µC/cm2 at room temperature,25 suggesting a compensating charge density of 1.15 e/surface unit cell when the thickness-dependent component of the dipole is perfectly screened. If the charges lie within a few Angstroms of the surface, the surface dipole will be on the order of 1 e-Å. Although the LiNbO3 pyroelectric coefficient is estimated to be only -0.004 µC/cm2-K near room temperature,26 the imbalance between the surface charges and the dipoles of the bulk repeat units rapidly leads to a large increase in the surface dipole; for the 0.5 mm thick samples used in this work, the increase would be 285 e-Å/K if no relaxation took place. As the temperature is ramped during the TPD experiments, therefore, huge fields could develop on the surface. In fact, such large fields created by the pyroelectric effect in LiNbO3 have been used to generate high-energy electron and ion beams under moderate vacuum.27 Therefore, if no relaxation occurs, a much more substantial effect than an increase in the activation energy for molecular desorption would be expected as the temperature is ramped. Although the field created by the pyroelectric effect can be large, the change in charge density on the surfaces required to screen the field is modest. For the entire 400 K TPD temperature ramp the change in the spontaneous polarization of LiNbO3 can be compensated for by changes in the charge densities on the opposing surfaces of less than 0.03 e/surface unit cell. Prior
13956 J. Phys. Chem. C, Vol. 111, No. 37, 2007 studies suggest that the electrical conductivity of LiNbO3 is too low for electron transport through the material to screen the field below 475 K.23 In the less than 10 min it took to record the TPD curves, adsorption of background gases in the UHV chamber can also be ruled out as a source of the additional charges. On the other hand, the surfaces were covered with 2-propanol molecules, and the screening could occur via decomposition and desorption of ionic fragments. Only a few hundredths of a monolayer of molecules would need to decompose, which would at best be near the TPD detection limit as illustrated in Figure 1 and is below the detection limit of our XPS system. If the screening lags the TPD temperature ramp, which would be expected since any screening mechanism is driven by a field building up on the surface, then the surface dipole would still be anticipated to change during the TPD experiments but not as dramatically as in the absence of any screening processes. It is interesting to compare the difference between the 2-propanol heat of adsorption on the positively and negatively poled surfaces (with the back-side roughened) to the activation energy to switch the polarization of a thin ferroelectric film. The activation energy to switch a single-domain ferroelectric thin film as a function of film thickness can be estimated from the coercive field, which gives the energy required to displace the charges, and the spontaneous polarization, which gives the number of charges that must be displaced per area. Using the room-temperature values of these parameters for stoichiometric LiNbO3, 50 kV/cm and 80 µC/cm2,25 yields a barrier of 5.7 meV/nm. Meanwhile, assuming unactivated desorption, the TPD data suggest an 11 kJ/mol or 0.11 eV higher heat of adsorption on the positive surface. If one molecule adsorbs per surface unit cell at saturation, then this energy difference would be sufficient to switch a nearly 20 nm thick film from negative to positive. Thus, the results suggest that differences in how strongly polar molecules interact with positively and negatively poled ferroelectric surfaces are large enough to make chemical sensors based on ferroelectric thin films feasible. 5. Summary Ferroelectric materials have drawn interest due to their spontaneous polarization and the ability to switch the polarization direction by applying an external electric field. In this paper, 2-propanol was used as a probe molecule to investigate the poling direction effect on adsorption and desorption of polar molecules. The saturation of 2-propanol on both the positively and negatively poled LiNbO3(0001) surfaces required exposures greater than 10 000 L at 190 K, indicating low initial sticking coefficients on the order of 10-4. The 2-propanol desorbed molecularly, independent of the poling direction. Molecular desorption from the positive surface, however, peaked at temperatures more than 100 K higher than the negative surface, indicating much stronger adsorption on the positive surface. In addition, the ferroelectric properties of the bulk led to unusual adsorption-desorption phenomena. These included abnormally low apparent desorption pre-exponentials (10 s-1 for the positive surface and 10-1 s-1 for the negative surface) and a strange sensitivity of the surface properties to the morphology of the back side of the crystal. The former was explained by the pyroelectric effect, i.e., the change in ferroelectric polarization
Yun et al. with temperature, creating a temperature-dependent heat of adsorption. It was found that a simple linear relation between the desorption activation energy and temperature fit the data well; using this relationship suggested that the desorption activation energy of 2-propanol from LiNbO3 increased by about 0.25 kJ/mol-K as the temperature was ramped during the TPD experiments. The implications of the results on the feasibility of creating ferroelectric chemical sensors were also evaluated. This analysis indicated that the 11 kJ/mol difference in heat of adsorption on the positive and negative surface is equivalent to the energy barrier to switch a roughly 20 nm thick LiNbO3, suggesting that differences in how strongly polar molecules interact with positively and negatively poled ferroelectric surfaces are large enough to make chemical sensors based on ferroelectric thin films feasible. Acknowledgment. This work was supported by the National Science Foundation under grant no. CHE-0413050. The authors also acknowledge the assistance of Prof. Charles Ahn and Prof. Victor E. Henrich and the Yale Materials Research Science and Engineering Center under NSF grant no. DMR-0520495 in carrying out this work. Lorenz Kampschulte thanks the German Academic Exchange Service (DAAD) for support while at Yale. References and Notes (1) Batchko, R. G.; Shur, V. Y.; Fejer, M. M.; Byer, R. L. Appl. Phys. Lett. 1999, 75, 1673. (2) Jin, B. M.; Bhalla, A. S.; Kim, I. W.; Chung, M. S. Ferroelectrics Lett. Sec. 1995, 19, 57. (3) Terabe, K.; Nakamura, M.; Takekawa, S.; Kitamura, K.; Higuchi, S.; Gotoh, Y.; Yasuo, C. Appl. Phys. Lett. 2003, 82, 433. (4) Wooten, E. L.; Kissa, K. M.; Yi-Yan, A.; Murphy, E. J.; Lafaw, D. A.; Hallemeier, P. F.; Maack, D.; Attanasio, D. V.; Fritz, D. J.; McBrien, G. J.; Bossi, D. E. IEEE J. Sel. Top. Quant. 2000, 6, 69. (5) Dulub, O.; Diebold, U.; Kresse, G. Phys. ReV. Lett. 2003, 90, 016102. (6) Fong, D. D.; Kolpak, A. M.; Eastman, J. A.; Streiffer, S. K.; Fuoss, P. H.; Stephenson, G. B.; Thompson, C.; Kim, D. M.; Choi, K. J.; Eom, C. B.; Grinberg, I.; Rappe, A. M. Phys. ReV. Lett. 2006, 96, 127601. (7) Giocondi, J. L.; Rohrer, G. S. J. Phys. Chem. B 2001, 105, 8275. (8) Inoue, Y.; Yoshioka, I.; Sato, K. J. Phys. Chem. 1984, 88, 1148. (9) Yun, Y.; Min, L.; Liao, D.; Kampschulte, L.; Altman, E. I. doi: 10.1016/j.susc.2007.08.001. (10) Weis, R. S.; Gaylord, T. K. Appl. Phys. A 1985, 37, 191. (11) Ching, W. Y.; Zong-Quan, G.; Yong-Nian, X. Phys. ReV. B 1994, 50, 1992. (12) Megaw, H. D. Acta Crystallogr. 1954, 7, 187. (13) Inoue, Y.; Watanabe, Y. Catal. Today 1993, 16, 487. (14) Idriss, H.; Andrews, R. M.; Barteau, M. A. J. Vac. Sci. Technol. A 1993, 11, 209. (15) Lahousse, C.; Bachelier, J.; Lavalley, J.-C.; Lauron-Pernot, H.; Le Govic, A.-M. J. Mol. Catal. 1994, 87, 329. (16) Mekhemer, G. A. H. Appl. Catal. A:Gen. 2004, 275, 1. (17) Fishel, C. T.; Davis, R. J. Langmuir 1994, 10, 159. (18) Brinkley, D.; Engel, T. J. Phys. Chem. B 2000, 104, 9836. (19) Wachs, I. E.; Chen, Y.; Jehng, J.-M.; Briand, L. E.; Tanaka, T. Catal. Today 2003, 78, 13. (20) Nakakura, C. Y.; Phanse, V. M.; Zheng, G.; Bannon, G.; Altman, E. I.; Lee, K. P. ReV. Sci. Instrum. 1998, 69, 3251. (21) Saito, A.; Matsumoto, H.; Ohnisi, S.; Akai-Kasaya, M.; Kumahara, Y.; Aono, M. Jpn. J. Appl. Phys. 1 2004, 43, 2057. (22) King, D. A. Surf. Sci. 1975, 47, 384. (23) Brownridge, J. D.; Shafroth, S. M. Trends in Lasers and ElectroOptics Research; Nova Science: 2006. (24) Noguera, C. J. Phys.: Condens. Matter 2000, 12, 367. (25) Gopalan, V.; Mitchell, T. E.; Furukawa, Y.; Kitamura, K. Appl. Phys. Lett. 1998, 72, 1981. (26) Savage, A. J. Appl. Phys. 1966, 37, 3071. (27) Brownridge, J. D.; Shafroth, S. M. J. Electrostat. 2005, 63, 249.
