Determination of Molecular Ordering at a Buried Interface and the

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Langmuir 2004, 20, 37-40

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Determination of Molecular Ordering at a Buried Interface and the Effect of Interfacial Ordering on Thin Film Crystallization by Second Harmonic Generation Minchul Yang† and Hai-Lung Dai* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 Received September 17, 2003. In Final Form: November 14, 2003 It is demonstrated that by using optical second harmonic generation the orientation and alignment of molecules in the interfacial layer between two solids, a thin solid molecular film and a metal substrate, can be determined. The pyridine molecules in the interfacial layer underneath the film are found to align along the [11 h 0] direction of the Ag(110) surface with a small tilt angle (∼11°) from the surface normal. This interfacial ordering is found to have a notable effect in inducing crystallization at the heterogeneous boundary of the amorphous molecular film.

The knowledge on the structure of the interfacial layer of atoms/molecules between two solids is important to a microscopic understanding of a variety of phenomena that involve an interface: adhesion, friction, charge and mass transport, melting, crystallization, and wetting, to name a few. This knowledge is becoming ever more significant as many newly developed areas of science and technology deal with systems of finite sizes where interfaces/ boundaries critically affect their properties. For example, molecular ordering at the solid/solid interface is crucial to achieving the orientation and proximity required for the electrical performance of thin film organic semiconductors.1,2 In the study of thin films on solid surfaces, it is intuitively assumed that the substrate surface structure affects the growth of the films. On a highly corrugated surface, the interfacial layer, that is, the layer of molecules at the bottom of the thin film, may assume a transitional structure between the surface and the bulk molecular solid. What would be the structure of this interfacial layer? How would it be influenced by the substrate surface? Would there be order within the interfacial layer even when the solid film is amorphous? Does the interfacial layer structure have any bearing on the continual growth of the film? These are important questions that need to be explored for understanding the growth of finite-size systems on solid substrates. Despite the strong and current interest in the description of the interfacial layer properties, a quantitative characterization of its structure has proven difficult because of a lack of experimental techniques that have sufficient sensitivity for the small number of interfacial atoms/molecules imbedded between two bulk phases. The surface-sensitive particle scattering techniques that require an ultrahigh vacuum (UHV) environment to operate usually have a penetration depth of only a few layers and are inept in dealing with an interface buried hundreds layers below. Photon-based linear spectroscopic techniques may have the penetration depth but are not discriminatory against bulk contributions. On the other hand, nonlinear † Present address: Department of Chemistry, University of California, Berkeley, CA 94720. * To whom all correspondence should be addressed at the Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323. E-mail: [email protected].

(1) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270. (2) Forrest, S. R. Chem. Rev. 1997, 97, 1793.

optical techniques such as second harmonic generation (SHG), because of the unique symmetry conditions, have both interface sensitivity and penetration depth for detecting a buried interface. Though nonlinear optical determination of the molecular orientation at interfaces between nonsolid phases, such as air/liquid,3-9 air/ solid,10-12 and liquid/liquid,13-15 has been performed, the detection of molecules at solid/solid interfaces has not been achieved. In this paper, we show the determination of the structure, including the tilt and azimuthal orientation angles and the alignment direction, of the molecules at a solid/solid interface by using SHG. Experiments reveal that the interfacial molecules, depending on the substrate structure, have structural order that affects thin film growth and structure. SHG determination of the interfacial structure should in general work for a system composed of molecules that individually have nonzero second-order nonlinear susceptibility. This first determination is performed for the interfacial layer of pyridine (C5H5N) molecules between a solid thin pyridine film and a Ag surface. Previous studies have shown that thin pyridine films with 10-100-nm thickness deposited on the Ag(111) surface at temperatures below 120 K were amorphous and produced no SHG.16 Upon annealing, this amorphous structure turns into a structure with domains of polycrystallites that give (3) Kemnitz, K.; Bhattacharyya, K.; Hicks, J. M.; Pinto, G. R.; Eisenthal, K. B.; Heinz, T. F. Chem. Phys. Lett. 1986, 131, 285. (4) Benderskii, A. V.; Eisenthal, K. B. J. Phys. Chem. B 2001, 105, 6698. (5) Zhuang, X.; Wilk, D.; Marrucci, L.; Shen, Y. R. Phys. Rev. Lett. 1995, 75, 2144. (6) Rasing, T.; Shen, Y. R.; Kim, M. W.; Grubb, S. G. Phys. Rev. Lett. 1985, 55, 2903. (7) Bakiamoh, S. B.; Blanchard, G. J. Langmuir 2001, 17, 3438. (8) Hsiung, H.; Rodriguez-Parada, J.; Bekerbaner, R. Chem. Phys. Lett. 1991, 82, 88. (9) Sato, O.; Baba, R.; Hashimoto, K.; Fujishima, K. J. Phys. Chem. 1991, 95, 9636. (10) Eisert, F.; Dannenberger, O.; Buck, M. Phys. Rev. B 1998, 58, 10860. (11) Higgins, D. A.; Byerly, S. K.; Abrams, M. B.; Corn, R. M. J. Phys. Chem. 1991, 95, 6984. (12) Hsiung, H.; Meredith, G. R.; Vanherzeede, H.; Popovitz-Biro, E. S.; Lahav, M. Chem. Phys. Lett. 1989, 164, 539. (13) Richmond, G. L. Chem. Rev. 2002, 102, 2693. (14) Grubb, S. G.; Kim, M. W.; Rasing, T.; Shen, Y. R. Langmuir 1988, 4, 452. (15) Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1993, 97, 489. (16) Sjodin, T.; Troxler, T.; Dai, H. L. Langmuir 2000, 16, 2832.

10.1021/la035744f CCC: $27.50 © 2004 American Chemical Society Published on Web 12/09/2003

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SHG.16 Temperature-programmed desorption spectroscopy reveals that the films, both annealed and unannealed, have an interfacial layer that has a distinct desorption energy from that of the film bulk, implying an interfacial layer structure different from that of the bulk.17 For the unannealed films where the amorphous structure gives no SHG, if molecules in this interfacial layer have orientation or alignment order, they will contribute to SHG. This additional source of SHG can be detected as interference to the metal-surface SHG and, thus, used for the determination of the structure of the molecules at the interface. All the experiments were performed in an UHV chamber with a base pressure of 1 × 10-10 Torr. Ag surfaces were cleaned by cycles of Ar+ sputtering and annealing to 700 K before each experiment. Pyridine (>99.5% anhydrous) was purified by freeze-pump-thaw cycles before use. For the nonlinear optical experiments, a 532-nm output from a Nd:YAG laser (Continuum 580A, 8-ns pulse length, 15 mJ/pulse in a 4.5-mm-diameter beam area, linearly polarized) was used as the fundamental light. The laser beam incident angle was 62° from the surface normal. Details of the experimental setup can be found elsewhere.16 To present the experimental observations, we use the notation of laboratory and crystal axes illustrated in Figure 1. The light incident plane is defined as the X-Z plane in the XYZ laboratory coordinates. The xyz axes represent the (110) surface coordinates with the [11h 0] and [001] directions assigned as the x and y axes. The crystallinesurface rotation angle with respect to the light incident plane is defined as φ. The orientation of the pyridine molecule at the interface is set by the tilt angle θ with respect to the surface normal and the rotation angle R between the x axis and the projection of the pyridine C2 axis onto the surface. In Figure 2a, second harmonic (SH) intensity from a clean Ag(110) surface measured as a function of the azimuthal angle φ is shown. The SH intensity displays a periodically oscillatory pattern that can be described by the nonlinear optical response of the crystalline metal surface with twofold symmetry.18 Under the dipole approximation, the p-polarized SH intensity generated with sp in Figure 2a, an s-polarized fundamental beam, I Ag(110) 18 can be described as sp I Ag(110) ) c3|1 + c0eic1 cos[2(φ - c2)]|2

(1)

where the parameter c0 depicts the relative contribution from the anisotropic part with respect to the isotropic part, c1 accounts for the relative phase of these two sources, and c2 is included to compensate for instrumental error related to the alignment of the φ angle. c3 is a proportional constant that depends on the Fresnel factors. The solid line in Figure 2a shows the nonlinear least-squares fit. The fitted parameters are listed in Table 1. The φ dependence of the SH intensity from the Ag surface deposited with an amorphous thin film should be identical to that of a clean Ag surface if the metal surface is the sole source of SHG. This indeed is the observation made on a thin amorphous pyridine film on Ag(111)16 but not for the case on Ag(110). The variation of the SH intensity with φ for an 85-nm-thick film grown amorphously on Ag(110), shown in Figure 2b, is apparently different from that of a clean surface shown in Figure 2a. (17) Yang, M. C.; Rockey, T.; Pursell, D.; Dai, H. L. J. Phys. Chem. B 2001, 105, 11945. (18) Sipe, J. E.; Moss, D. J.; Driel, H. M. Phys. Rev. B 1987, 35, 1129.

Figure 1. Schematic diagram of the laboratory (XYZ) and crystal surface (xyz) axes. βaaa represents the dominant hyperpolarizability element of pyridine along the molecular a axis.

Figure 2. Azimuthal angle dependence of the SH intensity for (a) clean Ag(110) and (b) an 85-nm-thick pyridine film on Ag(110). Filled circles are experimental results and solid lines are fitting results. Table 1. Values of the Parameters in Eqs 1 and 4 Extracted from Fittings Shown in Figure 2 with the Dielectric Constant for the Pyridine Layer Set as E1 ) 2.89 and for Silver as E2 ) 2.54 parameters (unit)

clean Ag(110)

c0 c1 (degree) c2 (degree) c3 c4 c5 (degree) c6 θ (degree) R (degree)

2.98 ( 0.04 93.68 ( 0.02 1.6 ( 2.4 0.074 ( 0.024

pyridine/Ag(110) 2.98 93.68 1.6 66 ( 24 66.7 ( 1.6 5.6 ( 2.4 11 ( 19 4.8 ( 1.0

The different pattern in Figure 2b must be a result of an additional contribution to the SH intensity from the film/metal interface. The contribution from the amorphous bulk film has been shown to be zero from the SH intensity dependence of the film thickness. The film/vacuum interface is not a likely source for additional SHG: if this were the case, this contribution should exist for both the (111) and the (110) systems because the structure of the uppermost layer of films with several hundred layers should be the same despite the difference in the underlying substrate structures. In the experimental observation, the azimuthal angle dependence of the SH intensity from the clean Ag(111) surface is not changed by the deposition of

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Langmuir, Vol. 20, No. 1, 2004 39

an amorphous pyridine film.16 Thus, we conclude that the additional source of SHG from the film/Ag(110) system must be from the film/metal interface. The pyridine-silver interaction is described as weak chemisorption17,19,20 where there is no substantial perturbation to either the electronic structure at the silver surface or the pyridine molecules. The effect of pyridine adsorption on the metal surface layer SHG is to decrease the magnitude of the nonlinear susceptibility of the bare metal surface, linear with pyridine coverage,21 due to either charge-transfer bonding or scattering of substrate electrons.21 Consequently, the SH intensity from the system with the two contributions to SHG, the pyridine interfacial layer and the metal surface layer, can be described as

I ) I0|γEAg(110) + eiδEpyr|2

(2)

where EAg(110) is the SH field from the Ag(110) surface, Epyr is that from the interfacial pyridine molecules, which is related to the ensemble sum of individual molecular polarizabilities, δ is the phase difference between the two fields, and γ is a constant with 0 < γ < 1. In our analysis, we assume that pyridine has one dominant element of hyperpolarizability, βaaa, along the molecular a-rotation axis (the C2 symmetry axis). Even though there has been no experimental or theoretical determination of the susceptibility tensor of pyridine at the energies of concern, this assumption is justified because of the symmetry of the moleculesthe inversion symmetry is broken along the a axissand because the strongest (by more than 1 order of magnitude) electronic transition dipole of the molecule is along the a axis.22,23 Epyr , originated from the ensemble sum of βaaa and detected in the specula reflection angle with p polarization, can be described by the three-media model, in which 1 and 2 are respectively dielectric constants of the interfacial pyridine layer and silver, as

Epyr ∝ βaaa sin2 φ sin2 θ[(2 - sin2 θ0)1/2 cos φ sin θ + (2/1) sin θ0 cosθ](Eωs )2 (3) where θ0 is the incidence angle of the fundamental beam with s-polarized electric field Eωs . The azimuthal angle dependence of the SH intensity in Figure 2b is then 2ω ) c6|1 + c0eic1 cos[2(φ - c2)] + I pyr/Ag

c4eic5 sin2(φ - R) sin2(θ)[(2 - sin2 θ0)1/2 × cos(φ - R) sin θ + (2/1) sin θ0 cos θ]|2 (4) The parameters c0-c2 in eq 4 are taken as the same as those in eq 1 and set at the same values obtained from the clean Ag(110) results. The other parameters can be extracted by a fitting of the data in Figure 2b. Parameter c4 depicts the relative contribution from interfacial pyridine with respect to any isotropic source, c5 accounts for the relative phase of these two sources, and c6 is a proportion constant. The extracted parameters are summarized in Table 1. From the nonlinear least-squares analysis, the angle θ is determined to be between 0 and 30° (one sigma) with (19) Avouris, P.; Demuth, J. E. J. Chem. Phys. 1981, 75, 4783. (20) Bader, M.; Haase, J.; Frank, K. H.; Puschmann, A.; Otto, A. Phys. Rev. Lett. 1986, 56, 1921. (21) Heskett, D.; Urbach, L. E.; Song, K. J.; Plummer, E. W.; Dai, H. L. Surf. Sci. 1988, 197, 225. (22) Yang, W.; Schatz, G. C. J. Chem. Phys. 1992, 97, 3831. (23) Bolovinos, A.; Tsekeris, P.; Philis, J.; Pantos, E.; Audritsopoulos, G. J. Mol. Spectrosc. 1984, 103, 240.

Figure 3. Graphical display of the determined orientation and alignment of the interfacial layer of pyridine molecules between the amorphous bulk film and the Ag(110) surface. The molecules and Ag atoms on the (110) surface are drawn according to scale.

the most probable value at 11°. In addition, it was found that the angle R is 4.8 ( 1.0°. The small R means that pyridine molecules are arrayed along the [11 h 0] direction such that the molecular plane is parallel to the [001] direction. The orientation and alignment of the interfacial molecules deduced from the SHG measurements are summarized in Figure 3. The SHG experiment could not determine if the alignment along the [11h 0] direction is in a zigzag arrangement or in one array. The lack of knowledge of 1 for the interfacial pyridine layer gives rise to uncertainty for the fitted parameters, which is determined by varying the value of 1 from 1 to 2.89. The latter is the dielectric constant of liquid pyridine that is usually the chosen value.10,24 The angle θ varied correspondingly between 6 and 11°, small in comparison with the fitting uncertainty. The angle R varied less than 1°. Electron energy loss spectroscopy and ultraviolet photoelectron spectroscopy experiments25 have deduced that pyridine molecules adsorbed on Ag(111) at near-saturation submonolayer coverage have an average tilt angle of 33 ( 3°. A near-edge X-ray absorption fine structure study20 determined the tilt angle as 20 ( 5°. These observations at the vacuum/metal interface are consistent with our discovery on the film/metal interfacial layer. The interfacial molecules stand up with their molecular planes close to the surface normal, presumably allowing the nitrogen lone pair to interact with the silver atom and intermolecular π-π interaction. It has been suggested that the combined adsorbate-substrate and interadsorbate interactions will result in the maximum potential energy in the saturated pyridine monolayer.25 The alignment of the molecules along the [11h 0] direction most likely results from the corrugation of Ag(110) and the accommodation of intermolecular interactions on this surface. The unit cell dimension on Ag(110) is 2.88 × 4.09 Å2. Aligning along the [11 h 0] direction where the Ag atoms are separated by 2.88 Å allows the pyridine molecules, with a van der Waals length of 6.64 Å along the a axis, to have the closest packing. Also consistent with our observation, pyridine molecules were found to align on Cu(110) with the a axis rotated 25-30° away from the [11 h 0] direction at saturation monolayer coverage.26,27 The larger rotation angle is probably due to the need to accommodate adjacent pyridine molecules on Cu(110), which has a shorter (2.55 Å) distance between Cu atoms along the [11 h 0] direction. A dramatic influence of substrate structure on interfacial molecular ordering is revealed in the comparison of (24) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. Rev. B 1999, 59, 12632. (25) Demuth, J. E.; Christmann, K.; Sanda, P. N. Chem. Phys. Lett. 1980, 76, 201. (26) Lee, J.-G.; Ahner, J.; Yate, J. T., Jr. J. Chem. Phys. 2001, 114, 1414. (27) Giessel, T.; Schaff, O.; Lindsay, P.; Baumgartel, P.; Polcik, M.; Bradshaw, A. M.; Koebbel, A.; McCabe, T.; Bridge, M.; Lloyd, D. R.; Woodruff, D. P. J. Chem. Phys. 1999, 110, 9666.

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the alignments of pyridine molecules on Ag(110) and Ag(111). According to Sjodin et al.,16 ss-polarized SH intensity from both clean Ag(111) and pyridine-filmcovered Ag(111) is well-described by the same sixfold symmetry pattern in azimuthal angle dependence. The lack of change in the SHG azimuthal angle dependence indicates that there is no additional SHG source from the interfacial pyridine molecules to interfere with the Ag surface SHG. On the basis of this observation,16 it was concluded that pyridine molecules at the film/Ag(111) interface on this least corrugated surface are not aligned on the surface plane even though they may have orientational (tilt-angle) order. The amorphous pyridine films deposited at 100 K can be annealed to form a crystalline structure.16 The effect of crystallization induced by annealing on SHG is shown in Figure 4. The SH signal at lower temperatures prior to annealing is from the Ag surface only. Upon annealing, the SH intensity of a 180-nm-thick pyridine film, deposited originally at 100 K on either (110) or (111), increases as the temperature is increased. In the case of the pyridine film on (111), the SH intensity shows a notable increase starting at 121 K. In contrast, the SH intensity change in the case of films on (110) shows two regimes: a slower increase starting at 112 K followed by a sharp increase passing 121 K. The slow increase of the SH intensity between 112 and 121 K is a feature not found for films on (111) and is a clear indication of an ordered structure in films on (110) in that temperature range. Because this increase occurs at temperatures before the bulk crystallization temperature of >120 K, it is attributed to surfaceinduced crystallization. Crystallization in the molecular film is initiated by the formation of critical-size nuclei of ordered molecular clusters. The interfacial pyridine layer on Ag(110) is already ordered at 100 K while the film bulk is amorphous. The ordered structure at the interface may serve as a nucleation center for crystallization. In general, hetero-

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Figure 4. Change of SH intensity during annealing of a 180nm-thick pyridine film on (a) Ag(110) and (b) Ag(111). The heating rate was 3 K/min.

geneous nucleation is more efficient than homogeneous nucleation so that crystallization at the interface may start at a temperature lower than that in the bulk. In contrast, in the case of films on (111), where the pyridine interfacial layer has no alignment order, there is no ordered structure to induce crystallization at the heterogeneous boundary. Here, only the bulk crystallization is observed. The contrast between the two surfaces provides a clear demonstration of the influence of the substrate structure on film crystallization. Acknowledgment. This work is supported by a grant from the Air Force Office of Scientific Research. The use of equipment supported by the National Science Foundation MRSEC Program No. DMR00-79909 is acknowledged. LA035744F