Bandgap engineering of germanene for gas sensing applications

Abstract

Gas sensors are used to detect gas components in human breath to diagnose diseases, such as cancers. However, choosing suitable two-dimensional materials for gas sensors is a challenge. Germanene can be a good candidate because of its outstanding electronic and structural properties. Based on the density functional theory calculations with various schemes, such as PBE + vdW-DF2, HSE06 + PBE, and HSE06 + vdW-DF2, we elucidated the structural and electronic properties of germanene substrates (perfect, vacancy-1, and vacancy-2) while adsorbing hepatocellular carcinoma-related volatile organic compounds (VOCs), i.e., acetone, 1,4-pentadiene, methylene chloride, phenol, and allyl methyl sulfide. These gases have been selected for investigation because of their most frequent occurence in diagnosing the disease. We found that vacancy substrates enhanced the adsorption strength of the VOCs compared to the perfect one, where the phenol adsorbed most strongly and exhibited the most profound influence on the structural deformation of the substrates over the other VOCs. Besides, the adsorbed VOCs significantly modified the energy bandgap of the considered germanene substrates. In particular, the gases, except allyl methyl sulfide, vanished the bandgap of the vacancy-1 germanene and converted this substrate from a semiconductor to a metal, while they widened the bandgap of the vacancy-2 structure compared to the isolated case. Therefore, the perfect and vacancy-2 germanene sheets could maintain their semiconducting state upon gas adsorption, implying that these substrates may be suitable candidates for gas sensing applications. The nature of the interaction between the VOCs and the germanene substrates is a physical adsorption with a weak charge exchange, which mainly comes from the contribution of the p_z orbital of the VOCs and the p_z orbital of Ge.

Fig. 1. The top view (upper panel) and side view (lower panel) of the optimal geometry of the isolated germanene sheets: perfect (a), vacancy-1 (b), and vacancy-2 (c).

Fig. 2. The optimized configuration of volatile organic molecules: (CH₃)₂CO (a), CH₂Cl₂ (b), C₄H₈S (c), C₅H₈ (d), and C₆H₆O (e). Oxygen (red), hydrogen (white), carbon (black), chlorine (green), and sulfur (yellow).

Fig. 3. The top and side views of the most favorable adsorption configuration of the VOC molecule on the germanene substrate: perfect (first column), vacancy-1 (second column), and vacancy-2 (last column). Ge/(CH₃)₂CO (a), Ge/CH₂Cl₂ (b), Ge/C₄H₈S (c), Ge/C₅H₈ (d), and Ge/C₆H₆O (e). The blue Ge atoms represent the adsorption sites of the VOCs.

Fig. 4. The adsorption energy for the most favorable configuration of the VOCs on germanene substrates.

Fig. 5. The band structure of germanene before and after the VOC adsorption by HSE06 + vdW-DF2 method. Perfect (top panel), Vacancy-1 (middle panel), and Vacancy-2 (bottom panel). Fermi level at 0 Ha.

Fig. 6. The comparison of the bandgap of the germanene substrates after the VOC adsorption by the PBE + vdW-DF2 (solid) and HSE06 + vdW-DF2 (dashed) methods.

Fig. 7. The total and orbital-projected density of states (TDOS and PDOS) of the gas-germanene systems by the HSE06 + vdW-DF2 method: the perfect germanene/VOCs (first row), vacancy-1 germanene/VOCs (second row), and vacancy-2 germanene (third row). The total density of states (gray area) of the Ge/VOC systems: (CH₃)₂CO (a), CH₂Cl₂ (b), C₄H₈S (c), C₅H₈ (d), and C₆H₆O (e).

Fig. 8. The charge density difference of the VOC-perfect germanene system in the most favorable adsorption configuration of the gases. Yellow represents occupied states, and cyan represents unoccupied states.