DFT Investigation of PFOS Interaction with 4H-SiC: A Feasibility Study for Environmental Sensing Applications

Perfluorooctanesulfonic acid (PFOS) is a persistent and toxic compound frequently detected in contaminated soil and groundwater. In this study, we employ density functional theory (DFT) to evaluate the potential of 4H-SiC, a low-cost, environmentally safe, and stable semiconductor, as a candidate sensing material for PFOS via surface adsorption and band gap modulation. Three configurations of deprotonated PFOS (vertical, inclined, and horizontal) were assessed on both Si- and C-terminated 4H-SiC slabs. The implicit effect of solvation (water) was included. In all cases, PFOS exhibited slight but energetically favorable adsorption. Electronic interactions were observed on the C-terminated slab, where PFOS adsorption induced midgap states and considerable band gap reduction. Among the configurations, vertical PFOS produced midgap states closest to the valence band maximum (VBM). In all cases, midgaps resulted from relatively weak interactions between surface carbon atoms and fluorine and carbon atoms in PFOS, as confirmed by analysis of the Kohn-Sham orbitals associated with the midgap energies. Localized surface states were more pronounced on the C-terminated slab compared with the Si-terminated one. Additionally, the planar-averaged potential difference between the two sides of the slab increased (compared to the pristine slab) upon PFOS adsorption on the C-terminated surface. Bader charge analysis revealed minimal electron transfer from PFOS to the surface. The band gap reduction of the C-terminated slab (e.g., from 2.58 eV (pristine) to 1.22 eV upon adsorption of vertically oriented PFOS) may suggest a promising sensing capability on the carbon-terminated surface. This study provides an initial assessment of PFOS interactions with 4H-SiC. Further DFT studies incorporating explicit solvent effects, alternative functionals, surface doping, and complementary experimental work are recommended to support the development of 4H-SiC-based PFOS sensor devices.