The electronic structure of the materials forming nanostructures plays a key role during chemical enhancement. It determines the excitation energy of the non-equilibrium charge carriers (hot electrons and holes) and their interaction with various functional groups of the organic molecules under investigation [1].

In the present work we discuss the interaction of an aqueous solution of tryptophan with thin polycrystalline films made from the intermetallic compounds Ag₃In and AgIn₂. Thin films with the required compositions (Ag/In ratios of 3:1 and 1:2) were prepared by layer-by-layer deposition alternating between very thin films of silver and indium in one vacuum cycle, and the compounds were formed through a self-occurring solid-state reaction. The chemical and phase compositions of the thin films, analyzed by energy-dispersive X-ray microanalysis and X-ray diffraction, confirmed the formation of intermetallic Ag₃In and AgIn₂ compounds.

The electronic states in the Ag-In nanostructures were analyzed by spectroscopic ellipsometry and using density functional theory calculations. The results show that the increased In content in the thin AgIn₂ films led to an increase in the energy for interband transitions from the Ag 4d electronic state to the Fermi level and to a respective increase in the energy and mobility of the generated hot holes. As a result of this, there was a decrease in the intensity of the peak at 1055 cm^-1, corresponding to the NH₂ group, and an enhancement of the peaks caused by the pyrrole and indole rings (situated at 1093, 1229, 1447 and 1490 cm^-1) in the surface enhancement Raman spectra of tryptophan.

These results show that by changing the electronic structure of plasmonic nanostructures, the orientation of the analyzed molecules and the chemical enhancement of the desired functional groups in them can be controlled.

[1]. Todorov, R. and Hristova-Vasileva, T., 2025. ACS Omega 2025, 10, 19, 19243–19255.

[2]. Todorov, R., Hristova-Vasileva, T., Katrova, V. and Atanasova, A., 2023. ACS Omega, 8(16), 14321-14341.