Program

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Thursday June 25

Th4: Tip-enhanced vibrational spectroscopies

Chair: J. I. Pascual, San Sebastián, Spain

Contributed talk

Determination of molecular orientation in monolayers adsorbed on metals using STM-based TERS

I. I. Rzeźnicka¹, H. Horino², N. Kikkawa¹, S. Sakaguchi¹, A. Morita^1,3, S. Takahashi⁴, T. Komeda⁴, H. Fukumura¹, T. Yamada⁵, and M. Kawai^5,6

¹Department of Chemistry, Graduate School of Science, Tohoku University, Japan

²Graduate School of Environmental Studies, Tohoku University, Japan

³Elements Strategy Initiative for Catalysis and Batteries, Kyoto University, Japan

⁴Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan

⁵RIKEN, Advanced Science Institute, Wako, Japan

⁶Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Japan

Understanding the orientation of organic molecules in thin films adsorbed on solid surfaces is of fundamental importance for the development of molecular devices such as biosensors or organic displays. Experimental techniques which can provide information on molecular orientation in the adlayers formed on metals have been limited to few techniques requiring an ultra-high vacuum environment, or polarization-dependent infrared studies. The unique feature of the scanning tunneling microscopy-based tip-enhanced Raman spectroscopy (STM-TERS) is its ability to obtain simultaneously molecularly resolved images of the adlayer structure and its vibrational signals [1].

In this talk, I will present our recent results on adsorption of N-heterocycles on metal surfaces using STM-TERS. I will show that information on molecular orientation in the adlayers formed on metals can be derived by comparison of the experimental scattering intensity data and theoretical calculation of Raman tensor elements.

Specifically, we investigated molecular adsorption and orientation of 4,4'-bipyridine (BiPy) and 4,4-bipyridine N,N-dioxide (BiPyO₂) in monolayers formed on Au(111). The results show two adsorption regimes for 4,4'-BiPy. At the initial phase of growth, a physisorbed phase is observed with molecules standing upright and with the molecular longitudinal axis parallel to the surface. At the later stage, molecules form a homogenous monolayer with pits of a single-gold-atom depth. An intense Au-N stretching signal indicated chemisorption of molecules to the metal surface. By comparison of the scattering intensities of TERS vibrational modes with the intensities calculated for three possible molecular orientations an upright molecular orientation with the molecular long axis perpendicular to the surface was concluded.

Adsorption of 4,4'-BiPyO₂ on Au resulted in formation of two-dimensional adlattice with the (6×9) symmetry. Neither Au-O nor Au-N vibrational modes were found for 4,4'-BiPyO₂ adlayer, indicating rather weak interaction with the gold surface. 4,4'-BiPyO₂ was adsorbed in a vertical configuration with the molecular longitudinal axis parallel to the surface.

These results demonstrate that STM-TERS is an effective method for simultaneous imaging and vibrational spectroscopy facilitating identification of the adsorbed layers at nano-scale spatial resolution with monolayer sensitivity [2].

Acknowledgements: This work was partly supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Grant No. 25390001), and 2011 Sunbor Grant from Suntory Foundation for Life Science.

[1] B.S. Yeo et al., Chem. Phys. Lett. 472, 1 ( 2009)

[2] I. I. Rzeźnicka et al., Surf. Sci. 617, 1 (2013)