Enhancing the gas sensing of porous silicon by surface modification using non-thermal plasma

This study used a non-thermal atmospheric argon plasma jet (NTAAPJ) as an innovative technique for treating and altering porous silicon surfaces (PSi) and enhancing their performance as nitrogen dioxide (NO₂) gas sensors. To prepare the PSi layers, we used photo-electrochemical etching (PECE). To treat the surface of porous silicon samples for different durations, the NTAAPJ was used at a plasma voltage of 18 kV. The influence of treatment on the structural and morphological properties of PSi was studied. X-ray diffraction (XRD) analysis revealed a shift in the (111) diffraction peak to a lower angle, indicating a reduction in internal microstrain. Additionally, the crystalline size increased from 28.6 to 54.6 nm after 16 min of treatment, suggesting enhanced crystal growth and improved structural ordering. Field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) images revealed an expansion of pore diameter from 1.1 to 2.4 μm and a porosity percentage increase from 11 to 75% after treatment. At the same time, surface regularity and roughness values improved, indicating a positive effect on surface structure and physical properties. Raman spectra further indicated improved crystallinity, with a blue shift in the Raman band and increased intensity as the time of treatment increases. Meanwhile, the Fourier transform infrared (FTIR) results illustrate that the plasma treatment did not alter the peak position or create new chemical bonds. Energy dispersive spectroscopy (EDS) analysis indicated that the PSi includes silicon, oxygen, and carbon. The oxygen percentage increased from 0.3% to 21.1%, and the carbon percentage decreased from 21 to 14% after treatment. The sensitivity of both treated and untreated samples to 60 ppm of nitrogen dioxide (NO₂) was evaluated at three distinct working temperatures: room temperature (RT), 75 °C, and 125 °C. The samples’ maximum sensitivity was recorded at 75 °C, indicating its superiority over other temperatures in gas detection. The results demonstrated a progressive improvement in sensing capability as the treatment period increased.