Charge-independent biomolecule detection using field-effect transistors (FETs) with single-crystal and large-area epitaxial graphene films fabricated on SiC substrates is demonstrated. To obtain clean graphene channel surfaces, FETs were fabricated using stencil mask lithography, which is a resist-free fabrication process. Proteins with various isoelectric points (pI: 5.6–9.9) were used as targets. Transfer characteristics [drain current (ID) vs solution-gate voltage (VG) characteristics] were measured by changing the pH of the buffer solution. The ID–VG characteristics exhibited a clear negative gate voltage shift for both positively and negatively charged proteins, indicating that the epitaxial graphene FETs could not detect the charge type of the protein and electrons were doped by the adsorption of both positively and negatively charged proteins. These results cannot be explained by conventional electrostatic effects. Therefore, it can be concluded that the detection of biomolecules by the epitaxial graphene FETs occurred through charge transfer from the proteins. Moreover, the dissociation constants between the proteins and epitaxial graphene films were as small as 100 pM, indicating the high sensitivity of the graphene FETs.
The effects of strong‐acid treatment on an epitaxial graphene film on a SiC substrate are investigated to confirm its stability and compatibility with conventional semiconductor device fabrication processes. An epitaxial graphene film is treated with a strong acid in the form of piranha solution (H2O2 + H2SO4), which is conventionally used in washing processes for the silicon‐based technology. Raman spectroscopy, Hall measurements, and contact angle measurements are carried out before and after piranha treatment. Raman mapping results show no drastic changes before and after piranha treatment. In particular, the D band is not observed after the piranha treatment. From Hall measurements, the electron mobility slightly increases from 920 to 1420 cm2 V−1 s−1 after five piranha treatments. The contact angle is almost constant before (72.8°) and after five piranha treatments (75.2°). These results indicate that the epitaxial graphene film is quite stable under piranha treatment.
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