High Sensitivity of Dry-Type Nanowire Sensors With High- $k$ Dielectrics for pH Detection via Capillary Atomic Force Microscope Tip Coating Technique

Dry-type poly-Si nanowire pH sensors with high-k dielectrics have been demonstrated with the aid of novel focus ion beam engineered capillary atomic force microscopy (C-AFM) tip. By means of this C-AFM tip coating technique, the relatively few testing solutions can be transferred onto the surface of a nanowire, preventing the sensor device from the immersion in the liquid and therefore suppressing the possible leakage current from the testing solution. As compared with the TEOS SiO₂, the pH sensors comprising Al₂O₃, TiO₂, and HfO₂ high- k materials exhibit the better sensitivities due to their enhanced capacitances. The best sensitivity (138.7 nA/pH) and linearity (99.69%) for a HfO₂ dielectric can be ascribed to the higher k value and larger bandgap with respect to the Al₂ O₃ and TiO₂, accordingly. Consequently, the C-AFM tip coating technique incorporating with HfO₂ dielectric suggests the potential for the detection of a minute quantity of biomedicines.