Atomic-scale engineering of MoSe2 via germanium doping for advanced detection of toxic alkaloids and alcohols

The urgent need for reliable sensors capable of detecting toxic alkaloids and alcohol-containing gases, including caffeine, nicotine, methanol, and ethanol, motivated our investigation into the structural, electronic, optical, and gas-sensing properties of the transition-metal dichalcogenide MoSe₂. Spin-polarized density functional theory (DFT) calculations show that alkaloids and alcohols interact weakly with pristine MoSe₂, limiting its potential as a reliable gas-sensing material. Elemental doping with Ge at a low concentration (2.08%) markedly strengthens the sensing capability of MoSe₂. Dispersion-corrected calculations show that substituting a single Se atom with Ge significantly enhances binding strength, with adsorption energies increasing from −0.21 to −0.96 eV on pristine MoSe₂ to −1.09 to −1.94 eV on Ge-doped MoSe₂, accompanied by pronounced electronic modifications. The optical response of Ge-doped MoSe₂ (MoSe₂@Ge) further demonstrates its suitability for gas sensing. Pronounced changes in dielectric function, absorption, reflectivity, and refractive index, particularly in the presence of nicotine and caffeine, highlight molecule-specific interactions, underscoring the material's potential for selective optical gas sensing. Mulliken charge analysis suggests that the enhancement in binding characteristics arises from a charge-transfer mechanism, where electrons are transferred from the alkaloids/alcohol (donors) to the MoSe₂@Ge surface (acceptor). Electronic structure analysis shows that Ge-doped MoSe₂ interacts weakly with methanol, ethanol, and caffeine via physisorption, resulting in modest bandgap increases and delocalized charge distributions. In contrast, nicotine exhibits strong chemisorption, accompanied by pronounced band-structure modifications, localized electron density, and the formation of a polar covalent bond. MoSe₂@Ge displays strong potential as a selective optical gas sensor, with distinct, measurable changes in its optical response upon gas adsorption. Nicotine and caffeine induce significant shifts due to stronger electronic interactions, whereas methanol and ethanol cause only minor effects. This work shows that ultra-low Ge doping converts MoSe₂ from a weakly interacting surface into a selective optical gas sensor, enabling molecule-specific discrimination between alkaloids and alcohols, particularly revealing strong chemisorption of nicotine versus physisorption of other gases, through distinct electronic and optical signatures.