Porosity-driven systematic modulations of surface plasmons in nanostructured metallic MoO2 films

Abstract

Metallic MoO<inf>2</inf> is known for strong plasmonic excitations, making it useful for surface-enhanced Raman spectroscopy (SERS). We developed a two-stage pulsed laser deposition (PLD) process to fabricate nanostructured MoO<inf>2</inf> thin films, while the conventional PLD typically yields continuous solid oxide films. With two-stage PLD, we induced uniform porosity to explore the effects of a porous surface on the plasmonic activities of MoO<inf>2</inf>. In the first stage of PLD, a MoO<inf>2</inf> seeding layer is deposited in the conventional manner, while the deposition angle was changed in the second stage to induce vertical porosity. The porosity varied depending on this angle, without losing uniformity of morphology. The porous nanostructures show weaker phonon–phonon interactions and reduced thermal conductivity as compared to compact films. Large plasmonic activity was observed for all the films, making these nanostructured MoO<inf>2</inf> films promising for SERS. With increased porosity, UV–Visible absorption spectra showed a pronounced blue shift and narrowing of the plasmonic resonance peak. We have achieved controlled modulation of surface plasmons by engineering the porosity here. Additionally, we demonstrated the use of these porous films for SERS by capturing the spectra of diluted methylene blue using these nanostructured MoO<inf>2</inf> films. The present results establish that PLD is an efficient tool, not only for the synthesis of solid films, but also for the growth of vertically porous yet crystallographically aligned nanostructured binary oxides, which is not yet achieved by any chemical method. © 2025