Theoretical Study on Structure and Sum-Frequency Generation (SFG) Spectroscopy of Styrene-Graphene Adsorption System Chih-Kai Lin,* ,†,‡ Chun-Chi Shih, † Yingli Niu, † Min-Yeh Tsai, † Ying-Jen Shiu, †,‡ Chaoyuan Zhu, † Michitoshi Hayashi, § and Sheng Hsien Lin †,‡ † Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010, ROC ‡ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 10617, ROC § Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan 106, ROC ABSTRACT: In this theoretical study, we aimed to simulate the sum-frequency generation (SFG) spectroscopy of a thin polystyrene layer physically adsorbed on the graphene sheet and to figure out the orientation distribution of the phenyl units. To simplify the problem, we started the investigation by constructing molecular models with styrene and ethylbenzene monomers and styrene oligomers up to four units adsorbed on a finite-sized graphene hexagon. Geometric optimization results showed that the phenyl rings of the adsorbate always orientate close to the surface normal with a small tilt angle. The adsorption is weak but not negligible. SFG spectra have been simulated based on these calculated structures, vibrational frequencies, and dipole and polarizability derivatives to compare with experimental reports of polystyrene adsorbed on other surfaces. I. INTRODUCTION The sum-frequency generation (SFG) spectroscopy has in recent years become a powerful technique to survey the surface or interface conformation of condensed materials. The achievement is attributed to its nonlinear optical character; that is, the spatial average of the second-order susceptibility, χ (2) , does not vanish in a noncentrosymmetric system. 1-4 On the surface or interface of materials, 7 of the 27 components in the third-rank tensor χ (2) survive. The peak intensities obtained from the SFG spectrum are then determined by the incident light beams as well as the spatial-averaged magnitude of these tensor components, which in turn are affected by the orientation of surface/interface molecules. The SFG spectroscopy, in principle, can be achieved by using any combination of two incident light beams. In practice, the resonance-off-resonance setting is mostly applied where the first (infrared) beam is tuned on resonance with a certain vibrational level of the molecular system, and the second (visible or ultraviolet) beam excites the system to a virtual electronic state. A resulted output beam with the sum frequency is then recorded when the incident beams are carefully arranged. By scanning the vibrational part and recording the output signals, a profile of surface or interface conformation would emerge. This technique has been successfully applied in studying configurations of the liquid water-vapor interface, 5-7 the ice surface, 2 organic adsorbate- substrate interfaces 8-11 as well as chiral compounds 4 and protein secondary structural motifs. 12 When a polymer molecule with side groups is adsorbed on a certain solid substrate, the orientations of these side groups should be determined by interactions between monomer units and between polymer molecule and substrate as well as the potential of free groups toward the air (or the vacuum). In case the adsorbed polymer film is thick enough, the configuration at the bottom (near the polymer-substrate interface) is expected different from that at the top (the polymer surface). People have explored, for example, the polystyrene (PS) film coated on solids and analyzed their SFG spectra to identify the tilt angle of the phenyl rings. Gautam et al. reported that the tilt angle was ∼20° at the PS/air interface and ∼70° at the PS/sapphire interface where the PS film had a thickness of 160 nm. 8 Briggman et al. suggested that, however, the angle was near 57° at the free surface in a PS-silicon system and in addition the phenyl groups on the surface were ordered and orientated outward. 9 In a more recent experiment, Han and Shen studied the PS-graphene system with SFG spectroscopy, yet they could not obtain a solid conclusion whether the phenyl groups tend to orientate to the surface normal or to tilt away. 13 In this theoretical work, we attempted to find out the most stable conformations of the styrene-graphene system and to simulate the SFG spectra. To simplify the problem, we made a systematic investigation starting from styrene monomer and ethylbenzene molecule adsorbed on a graphene sheet and then Received: October 17, 2012 Revised: December 13, 2012 Published: January 7, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 1754 dx.doi.org/10.1021/jp3102767 | J. Phys. Chem. C 2013, 117, 1754-1760