Surface-enhanced 2DIR spectroscopy of nm-thick films using plasmonic nano-arrays

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 Å, 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.