Layer-resolved characterization of subsurface of materials usmg ultrafast two-dimensional electronic spectroscopy

Abstract

Surface of matter normally contains sublayers with molecular or electronic structure different from the outmost surface and the bulk, which may play a critical role in surface energy and/or charge transfer processes. Therefore, the development of layer-resolved characterization methods is of great importance for surface science and techniques. Although optical spectroscopy methods are very sensitive to structure, their spatial resolution is often much larger than the inter-layer distance of the sublayers, resulting in the inability to achieve laminar resolution. In this work, we discuss the possibilities of utilizing two-dimensional (2D) electronic spectroscopy to distinguish spectral information and energy transfer between different layers, which cannot otherwise be obtained from linear spectroscopy methods owing to lineshape broadening. By theoretical 2D spectral simulations, we investigated two layered systems by numerical simulations, material surface:subsurface:bulk and molecule:surface:subsurface:bulk. The directional energy transfer rates from the bulk to the surface layer owing to the surface-bulk coupling was preset. Due to the fact that the energy transfer between the subsurface and the outermost surface layer is faster than that between the bulk and the outermost surface layer, there will be special time periods during which only the former takes place and gives rise to the distinguishable dynamics of off-diagonal peaks. Our simulation results showed that by analyzing the difference between the projections of the off-diagonal peak, the excitation energy of the surface, subsurface and bulk layers can be determined. Furthermore, for the four-layer system, the off-diagonal peaks can be kept away from the interference of diagonal peaks, providing a better chance for realizing laminar resolution compared with the three-layer system.