Figure 4.

MoS₂ FET and ReS₂ FET pH sensors. (A) Sensing mechanism of MoS₂ pH sensors. At low pH, ${\text{OH}}_2^{+}$ forms by protonation of OH on the dielectric surface, consequently forming a positive surface charge on the dielectric. At high pH, O⁻ forms by deprotonation of OH on the dielectric surface, consequently forming a negative surface charge on the dielectric. (B) Drain current vs. electrolyte gate voltage in various pH values for an n-type MoS₂ FET. Figures (A,B) were reproduced from Sarkar et al. (2014) with permission from the American Chemical Society. (C) Illustration of ReS₂ FET for pH sensing. (D) Transfer curves (I_DS-V_LG) at various pH values. (E) Response curves at different pH with V_DS = 0.1 V and V_LG = −0.5 V at RT. Figures (C–E) were reproduced from Liao et al. (2018) with permission from the American Chemical Society.