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 semicon- ductors. 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 descrip- tion 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 re- quire 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 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 sus- ceptibility. This first determination is performed for the interfacial layer of pyridine (C 5 H 5 N) 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 tempera- tures 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 † 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, Philadel- phia, PA 19104-6323. E-mail: dai@sas.upenn.edu. (1) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270. (2) Forrest, S. R. Chem. Rev. 1997, 97, 1793. (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. 37 Langmuir 2004, 20, 37-40 10.1021/la035744f CCC: $27.50 © 2004 American Chemical Society Published on Web 12/09/2003