Surface-Enhanced 2DIR Spectroscopy of nm-Thick Films Using Plasmonic Nano-arrays

Abstract

The ability to interrogate structure and dynamics of molecules at interfaces is crucial for many fields of science and technology. In this chapter, we discuss the implementation of surface-enhanced two-dimensional infrared (SE 2DIR) vibrational spectroscopy to examine molecules in thin films down to a monolayer, enabled by lithographically-fabricated plasmonic nano-antenna arrays. A variety of nonlinear IR measurements are presented for sample films of different thicknesses, ranging from a monolayer to ca. 100 nm, including diagonal and cross peak 3rd-order SE 2DIR measurements and diagonal 5th-order SE IR measurements. High raw-signal enhancement factors reaching 5.1 × 104 were obtained for diagonal peaks of a carbonyl stretching mode in a 1.5 nm thick sample via the 3rd-order SE 2DIR measurements. A similar enhancement factor of 4.3 × 104 was observed for cross peaks among the modes, frequency separated by ca. 350 cm−1 and distance separated by over 12 A, in nanometer-thick films. The mechanism of the enhancement was examined, using classical coupled oscillators approach. It revealed that for thicker samples the signal is dominated by a radiation damping mechanism, whereas the near-field enhancement is dominant for thinner samples of less than 3 nm thick. An application of SE 2DIR method to study sample monolayers and cross and diagonal peak waiting time dependences makes a wide arsenal of 2DIR techniques applicable for interrogating molecules at interfaces.